I did exactly the same, I couldnt get native working so I have a pbx inbetween, my inbound and outbound works perfectly, I just cant seem to get vapi to intitate a transfer from an assistant. I am not looking at a way I can use a SIP URI to my pbx (my provider wont allow it) to get transfers working. What I cant understand is why Vapi cant just send both inbound and outbound up and down the registered trunk??