What do you mean?
If you have GS phones you can add forward codes (from UCM) to phone.
Then if anyone set forward it will be set on UCM instead of phone.
I haven’t experience that issue, but take a look at your RTP settings.
the connection settings required back to your PBX would be the same … they are remote either way. Just confirming if they can connect when on the same network as the working desk phones.