I don’t recommend you use registration, just create a static definition.
Let’s assume you have a Asterisk 1.8 PBX at the IP 10.0.0.25 called ‘Kilroy’
Let’s assume your modern asterisk PBX is at the IP of 10.10.10.100 and it’s called ‘Bozo’
To define a trunk on Bozo to send and receive calls from Kilroy you would edit your pjsip.conf and create an aor, endpoint, and an identify section.
On Bozo you can dial extension 100 at Kilroy PBX by dialing PJSIP/100@kilroy
When calls arrive at Bozo from Kilroy they will be matched by the identify section to the endpoint and will arrive at the context ‘from-kilroy’
On Kilroy to create you trunk to Bozo you would modify sip.conf and add a peer.
To dial extension 200 at Bozo from Kilroy you would dial SIP/200@Bozo
When calls arrive at Kilroy from Bozo they will be matched by the IP address and will arrive at the context ‘from-bozo’