How to identify the type of call failure
Before troubleshooting, identify what the failure looks like. The category points you to the right section below and narrows the root cause faster.
If the failure looks like this... | Go to this section |
The call never connects | |
The call connects but audio is broken | |
The call is rejected or blocked | |
Failures started suddenly or only happen at higher volumes |
Common causes of call failures
Disconnected originating number
A call that returns a recording such as “you have reached a number that has been disconnected” points to an originating number that is no longer active or provisioned with Bandwidth. Confirm the number status in your Bandwidth account before retrying.
Issues with the terminating party
The terminating carrier or recipient may reject calls intentionally. Rejection can come from call blocking, carrier restrictions, or recipient-side configuration.
Unallocated or unassigned numbers
Calls to numbers that are unassigned or invalid fail at setup. Failures of this type usually trace back to dialing errors, routing issues, or outdated number data.
SIP signaling failures (4xx, 5xx, and 6xx response codes)
Session Initiation Protocol (SIP) response codes indicate why a call failed during setup or teardown. Use the tables below to interpret each class of response.
4xx client errors
A 4xx response points to dialing, authentication, or configuration problems on the originating side. Resolving a 4xx response usually means correcting credentials, number formatting, IP authorization, or routing configuration.
Code | Response | Common cause |
400 | Bad Request | Malformed SIP request |
401 | Unauthorized | Authentication failure |
403 | Forbidden | Request denied by policy or authorization |
404 | Not Found | Destination number not found |
408 | Request Timeout | No response within the expected window |
5xx server responses
A 5xx response points to network, routing, or capacity conditions on the server side.
Code | Response | Common cause |
500 | Server Internal Error | Internal server failure |
503 | Service Unavailable | Service unavailable, or expected LCR route-advance signal |
Important note for Wholesale Least Cost Route (LCR) customers: A SIP 503 response is not always a failed call attempt. For customers using a Least Cost Route (LCR) termination product, a 503 is an expected signaling response that tells your system to route-advance the call to the next available carrier in your routing logic.
In an LCR model, Bandwidth uses multiple upstream termination providers to deliver competitive rates. When a route is temporarily unavailable or cannot complete the call at the quoted rate, Bandwidth returns a 503 so your system can try another carrier.
Rate changes, capacity constraints, and upstream provider availability shift often, so 503 responses are expected behavior for LCR traffic. If your application cannot route-advance on a 503 and needs guaranteed completion, a different termination product may fit better.
Investigate 5xx responses further if any of the following apply:
They persist across retries
They occur across multiple destinations
Your traffic does not use an LCR-based termination model
6xx global failures
A 6xx response means the call was intentionally rejected and retries will not succeed.
Code | Response | Meaning |
603 | Decline | Call declined by the recipient or carrier |
607 | Unwanted | Recipient marked the call as unwanted |
608 | Rejected | Call rejected by an analytics or blocking service |
Audio and call quality issues
Audio issues appear after a call connects. Symptoms and causes often overlap, so check both lists before opening a ticket.
Common audio symptoms
One-way audio (only one party can hear)
Dropped calls
Choppy or distorted audio
Audio delay or echo
Typical audio causes
Network Address Translation (NAT) or firewall rules blocking Real-time Transport Protocol (RTP) media
SIP Application Layer Gateway (ALG) interfering with signaling
Packet loss, jitter, or general network instability
Codec mismatches between endpoints
If audio issues continue, a packet capture (PCAP) with RTP may be needed to pinpoint where audio is lost.
Intentionally blocked or restricted calls
Some call failures are intentional blocks, not technical errors. A blocked call will not succeed until the underlying restriction is removed — retrying without changes will not resolve the issue.
Common blocking scenarios
Invalid or unallocated Automatic Number Identification (ANI) on the originating number
Compliance or fraud-related restrictions
Country-based or destination-based restrictions
Short-duration calls flagged as suspicious
Capacity and congestion failures
Call failures can also occur when traffic exceeds your allowed limits. Review call volume, traffic patterns, and configured limits when failures appear only during peak usage.
Common capacity indicators
503 Service Unavailable responses during peak traffic
Failures that appear only under load
High call concurrency or calls-per-second violations
How to troubleshoot call failures step by step
Verify the originating number. Confirm the number is active and provisioned, check that the number is configured correctly in Bandwidth systems, and validate the number format (E.164 where required).
Check SIP response codes. Review SIP responses to identify client, server, or global failures. For Wholesale LCR traffic, confirm that any 503 responses match expected route-advance behavior.
Investigate the terminating side. Confirm the destination is not blocking or rejecting calls, and ask the terminating party to check with their carrier when needed.
Validate the destination number. Ensure the destination number is assigned and reachable, then place test calls from an off-net source to isolate routing issues.
Review network and media configuration. Disable SIP ALG, verify NAT, firewall, and RTP port settings, and check for packet loss or latency if audio issues are present.
When to contact Bandwidth Support
Contact Bandwidth Support when any of the following apply:
Failures are consistent and reproducible
5xx or 6xx SIP responses persist outside of expected LCR behavior
Calls fail across multiple destinations
Audio issues continue after network checks
When you contact Support, include the following details to speed up diagnosis:
Originating and destination numbers
Timestamps of failed calls
SIP response codes or error messages
A description of the observed behavior
Call failure troubleshooting summary
Call failures stem from four main sources: signaling errors, audio problems, intentional blocking, and capacity limits. Identifying the failure type first — and understanding how SIP response codes behave, including expected 503 responses in LCR environments — lets most issues be diagnosed and resolved quickly.