SMS Messaging: What it is, and how it works

What does SMS mean?

SMS stands for Short Message/Messaging Services, and is a type of text messaging for mobile phones and internet applications. 

SMS was established by the Global System for Mobile Communications (GSM) as a text-only protocol.


SMS is limited to sending plain text messages and is limited to 160 characters. MMS, which means Multimedia Messaging Services, can send videos, audio, images, and other media formats, including hyperlinks, to any device that is capable of receiving them (100% of iOS and Android devices can receive MMS). MMS also does not have the same character limits as SMS. Learn more about SMS vs. MMS.

SMS character limits & encoding standards

GSM-7 is a character encoding standard used for commonly used letters and symbols in many languages. It uses 7 bits to send a single character/symbol on GSM networks. As SMS messages are transmitted as 140 8-bit octets at a time, GSM-7 encoded SMS messages can carry up to 160 characters. 

On the other hand, UCS-2 is a character encoding standard used if a message cannot be encoded using GSM-7 or when a language requires more than 128 characters to be rendered. It uses a fixed length of 16 bits (2 bytes) to send a single character. As SMS messages are transmitted as 140 8-bit octets at a time, UCS-2 encoded messages can carry up to 70 characters. UCS-2 can encode anything in the Basic Multilingual Plane.

This limit was established because SMS was built to accommodate protocols that already existed. After SMS’s rise in popularity, however, the character limit was then enshrined in the SMPP Protocol. SMPP is how text messages are transmitted between carriers. However, most phones can break up and reassemble data packets into the original message format up to 1,600 characters.

Here’s the maximum number of characters that can be sent to carriers in a single SMS segment:

MessageTypeCharacters used in the messageEncodingMax characters/ message(without UDH)
Hello, good morning!TextGSM StandardGSM-7160

When converting your texts into a single data packet, those packets are binary data that includes your message and metadata such as the timestamp and the destination phone number. Before, you’d receive broken message fragments that would arrive out of order. Now, you can send and receive messages longer than 160 characters on your smartphone, which are delivered as one message. 

Longer messages are still broken into smaller packets, but through a truncation notation to the text’s metadata, the SMS packets can be reassembled and delivered as one message. Truncation depends on the delivering carrier, as some still do not support this. 

A history of SMS messaging

SMS messaging was first created as standard telecommunication infrastructure shifted from analog to digital in 1984. However, texting wasn’t introduced to the public until 1992, when engineer Neil Papworth sent the first-ever SMS on December 3. It said, “merry Christmas,” (And yes, the grammatical error is Mr. Papworth’s).

In the 1990s, texting services were expensive and constricted. Users paid per text message sent, and you could only text other users on the same carrier network. Nokia phones could send and receive texts, but the majority of phones could only receive texts. At this point, the average SMS messaging user was only sending 35 texts per month. It wasn’t until the 2000s that texting caught on, fueled by the emergence of cell phones and the reaction of the short code. 

How does SMS texting work?

When you send an SMS text, whether it’s from a phone, app, or other SMS-enabled mobile device, your message is translated into data and sent across an elaborate web of interconnected carrier networks. Your message will find the quickest path to the designated recipient. Once there, the data is translated back into a message and is delivered to the device.

When you hit “send,” that message packet gets transmitted via wireless through the Short Message Service Center (SMSC) to the nearest cell tower. Once there, the message finds its way to the recipient through the shortest route possible, landing at a tower near them and using the SMSC to deliver it to their mobile device. 

If the end recipient’s mobile device is turned on and in range, this only takes a few milliseconds. 


State of Messaging 2024

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Number types compatible with SMS messaging

Short codes 

Short codes are 5 to 6-digit numbers used to send high-volume, high-throughput A2P texts. While short codes can’t be used for two-way communications, certain keywords (such as “Yes” or “C” to confirm and “STOP” to opt-out) can be used as responses by recipients. Short codes cannot be used to make voice calls. 


Toll-free SMS allows for high-volume, high-throughput texts to be sent over toll-free numbers (800, 888, etc). Most existing toll-free numbers can be enabled to send SMS. Unlike short codes, toll-free SMS allows for two-way communications, allowing recipients to reply as they would to a normal text or call the number if it’s been voice-enabled.


10-digit long codes (10DLC) are sanctioned 10-digit, local phone numbers that are approved for high-volume, high-throughput messaging much like short codes or toll-free numbers. Like toll-free numbers, 10DLC numbers can handle two-way communications and voice calling, but similar to short codes, they have an associated carrier fee to access the A2P route. 

The messaging industry implemented 10DLC and this became widely available in 2020.

SMS message deliverability

Before an SMS can be delivered, it goes through several steps. After being converted into data, it’s sent across a web of carrier networks to the recipient. As each carrier receives the data, they review it to see if it’s SPAM. 

The criteria for whether or not a message is handed off to be delivered varies by each carrier, but as a general rule falls into four areas:

1. The Sender: Does the sender regularly send these types of messages? Are their messages regularly blocked?

2. How the message is being sent: Is the message being sent on the correct type of number? If the sender is using a P2P number to send A2P traffic, it can be blocked.

3. Message volume: based on the number being used, is the volume of messages correct?

4. The message content: Does the message contain illegal or disallowed content? Different types of URL shorteners can fall into this category and result in a message being blocked.

Error codes & messaging ROI

Calculating the ROI of your SMS efforts relies on the use of error codes. Different codes represent different types of reasons for a message not being delivered. This can include anything from a carrier blocking the message because of spam, to a number not being able to receive the text (ie: if a message is sent to a landline).

Using error codes, a business can calculate its delivery rate and ROI. It’s important to know that P2P SMS can only be tracked down to the carrier level. You’ll only know that the delivering carrier received the message; A2P can be tracked to the handset, giving you deeper insights into ROI.

The benefits of SMS


Thanks to the small size (140 bytes) and how SMS is transmitted, it takes only seconds for a text message to be delivered. This is especially valuable during times when phone lines are inaccessible or jammed due to high volume, such as during emergencies or natural disasters, as SMS can be queued and delivered as capacity becomes available, unlike a phone call.


Because of the lower bandwidth requirement associated with the small message files being exchanged, SMS can often be used in areas that might be out of range for voice transmission. Using an SMS gateway, messages can be sent and received via email in situations where there is no mobile service at all.


SMS is 100% compatible across all hardware and software platforms with as many as 98% of mobile device users regularly using SMS. Unlike proprietary messaging systems, which are dependent upon a particular operating system, or require a specialized application be installed in order to send and receive SMS messages, anyone with a modern mobile phone device of any kind can send and receive SMS.

Open & response rates

SMS is proven to have much higher open rates than other mass marketing methods. At a time when many ignore notifications from apps by swiping them away without reading them, users are more compelled to view an SMS message than they would be a mass email or telemarketing call, with SMS having a 98% open rate, compared to 21% open rate for emails and only 8% for push notifications.


Because of the universal cross platform nature, and its low bandwidth requirements, SMS is considered to be highly reliable. Due to the lower bandwidth needs, SMS messages can be delivered to recipients in fringe reception areas. If a recipient’s device is out of range or turned off, SMS messages will be delivered when the phone connects to its carrier network. For desktop SMS users, range is not an issue. People will often interact with SMS in situations where other forms of communication might be impractical. Compared to voice and email, SMS presents much more immediate and reliable connectivity.

Cost savings

For individuals, the rise of SMS has led carriers to move from a per message structure to flat fee pricing for unlimited messages. For businesses, SMS can help them realize savings by allowing them to remove or reduce operational costs, as there’s less human capital needed to manage SMS versus phone monitoring, which can be reduced to none if SMS responses are automated.

Flexible use cases

The ability to integrate SMS across platforms and its ubiquitous presence among consumers, gives businesses a level of flexibility and creativity that other communication methods don’t provide. The versatility of SMS is limited only by a business’ imagination, making it a powerful tool for communicating with customers.

How do businesses use SMS?

SMS vs. other types of messaging

SMS vs texting

There is no difference between SMS and texting. They are two words for the same thing—160-character plain text messages. However, many people refer to any messaging exchange as “texting” which confuses the landscape. Over-the-top applications like Facebook Messenger do not use SMS or MMS to exchange messages and should be considered messaging apps, not texting platforms. 


RCS, or Rich Communications Services, is a next-generation SMS protocol that upgrades text messaging but hasn’t seen the same adoption rate as SMS. RCS enables rich features like read receipts, suggested actions, high-quality images, and large videos without having to compress media. 

All Android devices are now being enabled to utilize RCS through Google Messages, but that will be proprietary to Androids, similar to iMessage for Apple. And in 2024, Apple plans to roll out RCS capabilities within iOS.

SMS vs iMessage

iMessage is proprietary to Apple and made for use with Apple devices. SMS is a cross-platform standard of sending messages, compatible with Android, Apple, and other platforms, and is compliant with Global System for Mobile Communication (GSM) standards. 

While iMessage natively integrates SMS into the messaging platform, so users can text between Apple and non-Apple users seamlessly, it is also a type of over-the-top (OTT) application that primarily utilizes cellular data or the internet to send messages between Apple iMessage users (think blue bubbles vs. green bubbles here, with green being SMS/MMS messages).

SMS vs Verified SMS

Verified SMS was an enhancement to standard SMS created by Google. Designed to provide an extra level of validation to texts sent by businesses, verified SMS confirmed that the message came from that business and would display that to the customer. Verified SMS has been retired and replaced with Google’s RCS Business Messaging. 

In its lifetime, verified SMS allowed businesses to send a branded experience, displaying the company logo in each message, and adding authenticity to the message’s origination. Verified SMS was intended to help reduce spam and allow customers who opt-in to receive SMS from a business know that the texts they received are legitimate. Verified SMS was only enabled in Google Messages, and was proprietary to Androids, similar to iMessage for Apple.

SMS vs WhatsApp

WhatsApp is a proprietary over-the-top (OTT) messaging application. While it runs across all software and hardware platforms, WhatsApp requires that both the sender and recipient have the application installed on their devices and access to cellular data or the internet to send messages. 

SMS does not require a separate application, and can be sent and received using default applications across multiple platforms with only regular wireless network access.

SMS guidelines & regulations

The CTIA (Cellular Telecommunications Industry Association) represents the US wireless industry and works to coordinate voluntary best practices and initiatives to maintain standards around SMS. Currently, those fall into two main categories: P2P and A2P.

Person-to-person SMS

Person-to-person SMS (P2P) is text messaging traffic that is typical of human-to-human communications. P2P should not exceed one (1) message-per-second (MPS), and the balance of messages sent versus received should be approximately one-to-one.

P2P SMS is defined as:

  • Messaging throughput of no more than 15-60 messages per second
  • Messaging volume of no more than 1,000 per day
  • A unique sender with only one telephone number assigned to or used by a single consumer.
  • Unique recipients with no more than 100 distinct receiving telephone numbers per message
  • A balance of a roughly 1:1 ratio for outgoing and incoming messages per telephone number
  • Repetition of 25 or more substantially repetitive messages will flag messages as A2P instead of P2P
  • Industries that were previously accepted for P2P use, including non-profits, political, and education services are now classified as A2P
  • Messages are strictly to be sent between consumers. Agents and businesses that demonstrate P2P traffic patterns are classified as A2P under these guidelines.

Application-to-person SMS

Application-to-person SMS (A2P) is text messaging that is sent from an application to a person and is generally classified as business messaging. Unlike P2P messaging, A2P allows for automated texts to be sent, which can exceed the one message-per-second rate that P2P is required to adhere to. A2P requires special numbers, including toll-free, short codes, and sanctioned local numbers known as 10DLC (10-digit long code).

A2P opt-in requirements

The CTIA defined three types of business messaging and laid out opt-in requirements for each one.

  1. Conversational messaging. This is consumer-initiated messaging. Consent is implied, so no additional opt-in is required.
  2. Informational messaging. This is when a consumer gives their phone number to a business and asks to be notified of account updates, appointments, and the like. Express consent is required, and the consumer may agree by completing a form via text, verbally, or on a website.
  3. Promotional messaging. This is any SMS promoting or marketing a business in any way. Express written consent is required for promotional messaging before a consumer can receive a text. Consumers can opt-in via a checkbox on a website, a signed form, or another web form that is continually tracked and managed.

These guidelines all exist to protect consumers and messaging as a channel for communication, both between individuals and between businesses and consumers.