Based on similar transfer issues reported by other users, here are the most common causes and troubleshooting steps:
Check your telephony provider configuration first(1). The most frequent cause of sudden transfer failures is an issue on your telephony provider’s side rather than with Retell(1).
Common issues to verify:
SIP 500 errors indicate your telephony provider’s server encountered an internal error(1). Check your provider’s logs and consider opening a support ticket with them(1).
Incorrect Termination URI in your phone number configuration can cause transfer failures(1). Your Termination URI should be the URI from your provider, not Retell’s URI(1).
Transfer destination routing problems may cause calls to loop back to the agent’s own number instead of the intended destination(2). This typically indicates incorrect routing rules in your SIP trunk or telephony provider configuration(2).
For warm transfers specifically, additional permissions from your telephony provider may be required(3). If cold transfer works but warm transfer fails, contact your provider about enabling the necessary permissions for warm transfers(3).
Verify your transfer call configuration:
- Ensure your transfer number is in E.164 format or a valid SIP URI format like
sip:username@domain(4)(5) - Check that your SIP trunk settings are correct, including termination settings for outbound calls and origination settings for inbound calls(1)
- Confirm authentication is properly set up with IP whitelisting or credentials(1)
For caller ID override issues, note that Retell-Twilio numbers support showing the user’s number on both warm and cold transfers, while Retell-Telnyx numbers only support this via SIP REFER with cold transfer(4)(5). If caller ID override isn’t supported by your provider, the transfer may fail(4)(5).
Check your call’s detailed logs for the specific disconnection reason and SIP error code to identify the root cause(1).