Roaming Extensions between U200 and S100 PBXs

Dear All,

I have the following scenario to allow my two extensions to ring at the same time:

I have extension number 1003 in U200 PBX, and another extension number 7089 in S100 PBX, the two PBXs are connected to each other via SIP VOIP trunk, and the two extensions (1003 and 7089) can call each other without any problem.

My request is, when other extension in any of the PBXs calls any of the two extensions, to ring the two extensions are the same time.

I tried it with the mobility extension in S100 but it did not work.

I contacted the support, they confirmed this feature is not available yet, and here is their reply:
1003 and 1010 are in the same U200, and 7089 is in the S100.
1003 calls 7089(mobility: 1010), the INVITE(caller with 1003) from S100 needs to be sent to U200(who has the 1003 extension). 
In this case, U200 requires S100 for 1003's authorization, which is not available now.

I appreciate if there is a solution for my case asap please.

Thanks

Tamer Shaarawy