FS is a B2BUA not a proxy. It doesn't forward messages to another server.

Although I generally agree with your statement. It is not completely accurate. The fact that FS is a B2BUA does not preclude it from eventually supporting this feature that is known sometimes as 'upper' or 'thru' registration, which is fairly common in SBCs, some of which are still B2BUA. There is little to no standard information in what a B2BUA can accomplish, so 'forwarding' messages is something a specific B2BUA implementation can do.

Sangoma's SBC, which has FreeSWITCH at its core has implemented this functionality. We hope to be able to merge it back as soon as we have a maintenance window that allows us to upgrade our patch to mod_sofia to 'forward' REGISTER/SUBSCRIBE/NOTIFY instead of handling it locally. This is similar to what a proxy would do, but not exactly, as separate dialogs are kept on each side (different from/to tags, call-id)

Cheers,

Moy