Outbound Caller ID: <blank>
Never Override CallerID: checked
Maxium channels: 10

Dial Rules:
1+NXXNXXXXXX
1212+NXXXXXX (where 212 is your area code)

Outbound Dial Prefix: <blank>

Trunk Name: callwithus

PEER Details:
context=from-trunk
host=sip.callwithus.com
qualify=no
username=username
secret=password
type=friend
insecure=invite

USER Context: <blank>
USER Details: <blank>

Register String: username:password@sip.callwithus.com

Dial command (extensions.conf):

_X. => 1,Dial(SIP/callwithus/${EXTEN},120)
_*X. => 1,Dial(SIP/callwithus/${EXTEN},120)

If you defined a DID in trixbox configuration as 1234567890, then set the DID destination to
SIP/username/1234567890.