# Call Not Transferring **URL:** \ **Category:** Support Help\ **Created:** [February 5, 2026, 7:03pm UTC](https://community.retellai.com/t/call-not-transferring/779 "2026-02-05T19:03:58Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Christian\_Torgerson](https://yyz2.discourse-cdn.com/flex008/user_avatar/community.retellai.com/christian_torgerson/32/354_2.png) [@Christian\_Torgerson](https://community.retellai.com/u/Christian_Torgerson)\ **Post date:** [February 5, 2026, 7:03pm UTC](https://community.retellai.com/t/call-not-transferring/779/1 "2026-02-05T19:03:58Z")
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 ---
**Author:** ![support2](https://yyz2.discourse-cdn.com/flex008/user_avatar/community.retellai.com/support2/32/15_2.png) [@support2](https://community.retellai.com/u/support2)\ **Post date:** [February 5, 2026, 7:04pm UTC](https://community.retellai.com/t/call-not-transferring/779/2 "2026-02-05T19:04:26Z")
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)](https://docs.retellai.com/build/conversation-flow/call-transfer-node)[(2)](https://docs.retellai.com/build/single-multi-prompt/transfer-call). For **cold transfer** , Retell uses SIP REFER, while **warm transfer** uses SIP DIAL[(2)](https://docs.retellai.com/build/single-multi-prompt/transfer-call)[(3)](https://docs.retellai.com/build/telephony/sip-headers). The transfer type you configure determines which method is used[(1)](https://docs.retellai.com/build/conversation-flow/call-transfer-node)[(2)](https://docs.retellai.com/build/single-multi-prompt/transfer-call). 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)](https://docs.retellai.com/deprecation-notice/1-26-2026). ## Caller ID and SIP Headers You can configure which caller ID shows up to the transfer destination[(1)](https://docs.retellai.com/build/conversation-flow/call-transfer-node)[(2)](https://docs.retellai.com/build/single-multi-prompt/transfer-call): - **Retell Agent’s number** : The transfer destination will see the Retell agent’s number[(1)](https://docs.retellai.com/build/conversation-flow/call-transfer-node)[(2)](https://docs.retellai.com/build/single-multi-prompt/transfer-call) - **User’s Number** : The transfer destination will see the number of the user[(1)](https://docs.retellai.com/build/conversation-flow/call-transfer-node)[(2)](https://docs.retellai.com/build/single-multi-prompt/transfer-call) For warm transfer, Retell sets `from` and `P-Asserted-Identity` headers to the user’s number[(2)](https://docs.retellai.com/build/single-multi-prompt/transfer-call)[(3)](https://docs.retellai.com/build/telephony/sip-headers). For cold transfer using SIP REFER, it’s up to the telephony provider to support caller ID override[(2)](https://docs.retellai.com/build/single-multi-prompt/transfer-call)[(3)](https://docs.retellai.com/build/telephony/sip-headers). You can also add custom SIP headers (must start with `X-`) for call transfers[(3)](https://docs.retellai.com/build/telephony/sip-headers). However, for cold transfer with transferee number using SIP REFER, different telephony providers may or may not honor the custom SIP headers[(3)](https://docs.retellai.com/build/telephony/sip-headers). ## 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)](https://docs.retellai.com/build/conversation-flow/call-transfer-node)[(2)](https://docs.retellai.com/build/single-multi-prompt/transfer-call) - Your caller ID configuration settings[(1)](https://docs.retellai.com/build/conversation-flow/call-transfer-node)[(2)](https://docs.retellai.com/build/single-multi-prompt/transfer-call) - If you need to explicitly set `cold_transfer_mode` to `sip_invite` instead of the default `sip_refer`[(4)](https://docs.retellai.com/deprecation-notice/1-26-2026) For detailed debugging, you may need to contact Retell support directly: - Visit [Contact Sales](https://retellai.octolane.com/) - Ask questions on [Discord](https://discord.com/invite/wxtjkjj2zp) - Visit [https://github.com/RetellAI/](https://github.com/RetellAI/) ---
**Author:** ![Omar\_Ashraf](https://yyz2.discourse-cdn.com/flex008/user_avatar/community.retellai.com/omar_ashraf/32/102_2.png) [@Omar\_Ashraf](https://community.retellai.com/u/Omar_Ashraf)\ **Post date:** [February 5, 2026, 9:02pm UTC](https://community.retellai.com/t/call-not-transferring/779/3 "2026-02-05T21:02:11Z")
Hello @Christian_Torgerson Can you share a call id that has this issue?