Call Not Transferring

Support Help
1

Hi Retell Support,

We’re having an issue with SIP transfers to Ringba.

We connected a Ringba SIP URL in Retell, but the call does not transfer. The same exact SIP URL works correctly in Vapi, so the endpoint itself is valid.

From Retell, the transfer attempt fails and the call does not reach Ringba.

Can you confirm:

  1. Whether Retell is sending this transfer as a SIP REFER or as a new outbound SIP INVITE

  2. Whether any SIP headers (From / Caller ID / P-Asserted-Identity / Diversion) are being stripped or defaulted to anonymous

  3. What codecs and DTMF modes are being used on the transfer

  4. If there are any known limitations with Ringba SIP endpoints

2

Based on the available documentation, here’s what I can confirm about Retell AI’s SIP transfer behavior:

Transfer Method (SIP REFER vs SIP INVITE)

Retell AI supports both cold transfer and warm transfer methods(1)(2). For cold transfer, Retell uses SIP REFER, while warm transfer uses SIP DIAL(2)(3). The transfer type you configure determines which method is used(1)(2).

As of the deprecation notice dated 01/23/2026, there’s a new cold_transfer_mode parameter that explicitly lets you choose between sip_refer and sip_invite for cold transfers(4).

Caller ID and SIP Headers

You can configure which caller ID shows up to the transfer destination(1)(2):

  • Retell Agent’s number: The transfer destination will see the Retell agent’s number(1)(2)
  • User’s Number: The transfer destination will see the number of the user(1)(2)

For warm transfer, Retell sets from and P-Asserted-Identity headers to the user’s number(2)(3). For cold transfer using SIP REFER, it’s up to the telephony provider to support caller ID override(2)(3).

You can also add custom SIP headers (must start with X-) for call transfers(3). However, for cold transfer with transferee number using SIP REFER, different telephony providers may or may not honor the custom SIP headers(3).

Codec and DTMF Information

The available documentation doesn’t specify the exact codecs or DTMF modes used during transfers.

Ringba-Specific Limitations

There are no known limitations with Ringba SIP endpoints mentioned in the documentation.

Troubleshooting Recommendations

Since your SIP URL works with other platforms, the issue likely relates to how Retell formats the transfer request. Check:

  • Whether you’re using cold or warm transfer mode(1)(2)
  • Your caller ID configuration settings(1)(2)
  • If you need to explicitly set cold_transfer_mode to sip_invite instead of the default sip_refer(4)

For detailed debugging, you may need to contact Retell support directly:

3

Hello @Christian_Torgerson

Can you share a call id that has this issue?