### Answer --- Yes, the transfer call feature works with custom telephony setups. [(1) ]() The documentation states that transfer call "is available for Retell numbers and imported numbers." When using elastic SIP trunking (the recommended custom telephony method) , "all the telephony functionalities that are supported by Retell numbers will also be supported here, assuming that your telephony provider supports it." [(2) ]() For the "Dial to SIP URI" method specifically, the FAQ confirms: "Will I be able to transfer call using SIP trunking? Yes you can use the transfer call feature with SIP trunking." Key considerations: - If you intend to use SIP REFER (cold transfer with the transferee's number) , you'll need to configure your SIP trunking provider to allow SIP REFER and PSTN transfer. - You can set the transfer destination as a number in e.164 format or a SIP URI in the format

sip:username@domain

. [(1) ]() - If your transfer destination is not in e.164 format, you can choose "raw format" to keep the input as-is—this only applies when using custom telephony. [(3) ]() - You can also add custom SIP headers for transfers (must start with

X-

) , though note that some providers like Twilio may not honor custom SIP headers in REFER requests for cold transfers. [(5) ]()

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

​ To learn more, see the sources I considered: [[1] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[2] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[3] Call Transfer Node](https://docs.retellai.com/build/conversation-flow/call-transfer-node) ​

@AI Evy sory, im using like the registeer call API to start calls. does it work with that?

### Answer --- Yes, transfer call should work when using the Register Phone Call API with the "Dial to SIP URI" method. [(1) ]() The documentation confirms: "Will I be able to transfer call using SIP trunking? Yes you can use the transfer call feature with SIP trunking." When you register a call via the

/v2/register-phone-call

endpoint and connect your PBX to the returned SIP URI, the agent's configured tools (including transfer call) remain functional. [(2) ]() , [(3) ]() Things to keep in mind: - Your telephony provider/PBX must support the transfer method being used (SIP REFER for cold transfer, SIP DIAL for warm transfer) [(1) ]() - If using SIP REFER, you'll need to configure your provider to allow SIP REFER and PSTN transfer - For raw/non-e.164 transfer destinations, you can select "raw format" in the transfer call tool settings—this only applies when using custom telephony [(3) ]()

​ To learn more, see the sources I considered: [[1] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[2] Register Phone Call](https://docs.retellai.com/api-references/register-phone-call) [[3] How Get only call_id, for dial to sip?](https://discord.com/channels/1186424376300679209/1385238437413654578) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.