this is in the documentation: SIP REFER (call transfer) If you need to transfer a call to another number, you will need to add a SIP Transfer based call forwarding where the transfer number will look like this: sip:transfer-number@your-telecom-provider-domain.com Example: sip:15557654321@sip.zadarma.com Note: Certain providers require phone numbers to be formatted in the proper E.164 standard. For example, the transfer URI should appear as: sip:+15557654321@sip.zadarma.com.  Example tool configuration required for SIP REFER: { "type": "transferCall", "destinations": [ { "type": "sip", "sipUri": "sip:14039932200@sip.telnyx.com" } ] } You might need to enable SIP REFER in your SIP provider to allow this.