We have a FreePBX where an inbound call (via IVR) shall be unconditionally transferred to an external number.
The problem is that the SIP-trunk provider does not accept the transferred call since the From-Field in the INVITE does not contain a number that is ‘owned by the trunk provider’.
What we need is to indicate that this INVITE is a ‘transferred call’ by using “REFER” or “302 Moved” in the INVITE so that it can be accepted by the trunk-provider.
I’d add another outbound trunk to a provider that allows you to set the From-Field to whatever you want. You don’t need a number with that provider - just set up an outbound connection.
Set up the IVR so that it sends the call to that outbound and off you go.