http://www.draytek.com/user/SupportAppnotesDetail.php?ID=877
The PBX looks looks at the INVITE line and uses the part before the @ sign in the address.
E.G.
Request 1
INVITE sip:800800@draytel.org
Request 2
INVITE sip:02083815500@217.1.2.3
Request type 1 would be answered by SIP trunk matching 800800
Request type 2 would be answered by SIP Alias matching 02083815500
SIP Alias' are setup in the Trunk menu.