Asterisk sip trunk to another asterisk

Hi. I have a main asterisk where all providers are on it and all of my users with extension numbering 3XX. There is a remote asterisk machine of a collegue that for business reasons i need to ggive them some DIDs routed from my PBX. I thought to give them an extension from my PBX and route the DIDs to that PBX.
Unfortuately it cannot register. I have tried the account in a zoiper softphone in their premises and it seems to work. So it is not a port forwarding issue.
I do not want to give them full access to my PBX with an IAX trunk. I would like to give them only a sip trunk to their PBX.
Some help with the settings please.