Does anyone know if it’s possible to set up ringing before the voice agent picks up, and then dial, through something like TwiML bin, an SIP trunk/route the call through an SIP trunk as though I were following the twilio inbound SIP trunk setup docs. So that unlike if I just dial the sip directly, SIP Refer is enabled, for the sake of tranferCall to numbers still working?

Yes, you can use a TwiML Bin to play ringing before connecting to a SIP endpoint via Twilio, and if routed through an Elastic SIP Trunk with SIP REFER enabled in its settings, this allows features like transferCall to work, unlike direct SIP dialing which doesn’t support REFER.

Do you know how I an Elastic SIP trunk?

Use sip:username@your-trunk.pstn.twilio.com, replacing with your trunk domain and user, ensure your trunk is set up with proper termination settings and supported codecs like G.711U.

Thanks mate, you’re a legend

Route calls from Zoiper through a SIP infrastructure like a PBX that connects to Twilio, and use Twilio’s free test numbers example +1 650-489-4546 to verify setup, ensuring IP-based authentication and G.711 codecs are properly configured.

Wehn you say domain, is it referring the termination domain? and what do you mean by the username?

Yes, the domain refers to your trunk’s Termination SIP URI, and the username is the user part of the SIP URI used by your SIP infrastructure to route or authenticate the call..

where can I find the username? I don't believe I've set one. My only point of reference would be the orgination uri which is sip:+1XXXXXXXXXX@sip.vapi.ai