Hi,
This is the way to do it. I would have avoided from answering the call. Just send the call to your sip provider.
Example:
[cust-fwd]
exten => _X.,1,Dial(SIP/${EXTEN}@siptrunk.com,60)
exten => _X.,n,Hangup
Thank you,
Daniel Friedman
Trixton LTD.