Call failures can occur for many reasons, ranging from dialing and configuration issues to network problems or intentional call blocking. This guide explains the most common types of call failures, how to identify them, and what steps to take to troubleshoot effectively.

Start here: Identify the type of failure
Before troubleshooting, determine what the failure looks like:
The call never connects → See SIP signaling failures
The call connects but audio is broken → See Audio and call quality issues
The call is rejected or blocked → See Blocked or restricted calls

Identifying the category first helps isolate the root cause faster.
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Common causes of call failures
Disconnected originating number
If calls return a message such as “you have reached a number that has been disconnected,” the originating number may no longer be active or provisioned with Bandwidth.
Issues with the terminating party
The terminating carrier or recipient may be rejecting calls intentionally. This can occur due to call blocking, carrier restrictions, or recipient-side configuration.
Unallocated or unassigned numbers
Calls to numbers that are unassigned or invalid will fail. This may be caused by dialing errors, routing issues, or outdated number information.
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SIP signaling failures (4xx, 5xx, 6xx response codes)
SIP response codes indicate why a call failed during setup or teardown.
How to interpret SIP response codes
4xx client errors
Typically caused by dialing, authentication, or configuration issues.
Common examples include:

These usually require correcting credentials, number formatting, IP authorization, or routing configuration.
5xx server responses
These indicate network, routing, or capacity-related conditions.
Common examples include:

Important note for Wholesale (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 instructs your system to route advance the call to the next available carrier in your routing logic.
In an LCR model, Bandwidth leverages multiple upstream termination providers to achieve competitive rates. If a route is temporarily unavailable or cannot complete the call at the quoted rate, Bandwidth will return a 503 to signal that another carrier should be attempted. Rate changes, capacity constraints, and upstream provider availability can vary frequently, making this behavior expected for LCR traffic.
If your application is unable to route advance on a 503 and requires guaranteed completion behavior, other termination products may be more appropriate.
Investigate 5xx responses further if:

6xx global failures
These indicate the call was intentionally rejected and retries will not succeed.
Common examples include:

Audio and call quality issues
These issues occur after a call successfully connects.
Common symptoms

Typical causes

If audio issues persist, packet capture (PCAP) analysis with RTP may be required to determine where audio is being lost.
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Blocked or restricted calls
Some call failures are caused by intentional blocking rather than technical errors.
Common blocking scenarios

Blocked calls will not succeed until the underlying restriction is removed. Retrying the call without changes will not resolve the issue.
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Capacity and congestion issues
Call failures may occur when traffic exceeds allowed limits.
Common indicators

Review call volume, traffic patterns, and configured limits if failures appear only during peak usage.
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Steps to troubleshoot call failures
1. Verify the originating number

2. Check SIP response codes

3. Investigate the terminating side

4. Validate the destination number

5. Review network and media configuration

When to contact Bandwidth support
Contact Bandwidth support if:

When contacting support, include:

Related topics

Summary
Call failures can stem from signaling errors, audio issues, blocking, or capacity limits. By identifying the failure type first and understanding how SIP response codes behave, including expected responses such as 503 in LCR environments, most issues can be diagnosed quickly and resolved efficiently.

Tim Hamill [4:10 PM]
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