----------------------------- **While I generate an answer, here are some sources that might be helpful**: * [Call Forwarding]() * [Function Calling]() * [Inbound Support Example]() * [Create Phone Call]() -----------------------------

### Answer To address the issues you're experiencing with call transfer functionality on Vapi, it's important to ensure that your setup is aligned with Vapi's capabilities and configurations as outlined in the documentation. Based on your description, you are attempting to use the

transferCall

function with a

sipUri

. The

transferCall

function, as described in the Vapi documentation, allows for the transfer of calls to a specified forwarding number when such a number is predefined in the assistant's configuration. To effectively utilize this function, make sure that your assistant's configuration is properly set up to include the

forwardingPhoneNumbers

or a

forwardingPhoneNumber

attribute that you wish to transfer calls to [(1)]() . For calls that originate from SIP, specifying a SIP URI (

sipUri

) in the

forwardingPhoneNumbers

array is crucial, as Vapi currently supports transferring calls to SIP URIs through the SIP REFER method [(2)]() . It's also important to ensure that the SIP URI format is correct and that your receiving system is configured to accept SIP REFER requests. If you are encountering issues despite this setup, it might be useful to review the implementation details and ensure that there are no discrepancies in how the

transferCall

function is invoked in your prompt and how the

forwardingPhoneNumbers

are defined in your configuration. Additionally, verifying that the receiving system (in this case, possibly Twilio given the SIP URI format) is correctly configured to handle incoming SIP REFER requests could help resolve the issue. Remember to test your setup thoroughly after making any adjustments to ensure that the call transfer functionality operates as expected. If problems persist, reviewing Vapi's logs and the configuration of the external sy

stems involved in the call transfer process for any errors or misconfigurations may provide further insights. For detailed information on Vapi's call forwarding and function calling capabilities, please refer to the provided documentation on [Call Forwarding]() and [Function Calling]() [(2)]() [(1)]() .

Got it. Tagging @User for additional help.