Hi there
Basically a SIP Trunk is normally a VoIP Connection that does NOT use Registration for authentication/authorization, but binds to a static IP Address and assumes that that IP address is authorized.
As Mike pointed out, you may be able to achieve the scope by configuring the other SIP endpoint as a VoIP Provider, and setting it to NOT require registration for both inbound and outbound calls.
You can then direct calls across this route via the Outbound Rules as with other VoIP Provider accounts. If you do not need/want to accept inbound calls over this channel, you may disable incoming calls for it. Some tweaking of the Outbound Parameters, Inbound Parameters, and Source Identification rules may be necessary.
Regards
Kevin