Multicast Paging Across 3 Facilities?

[VOIP_IT_Guy]

I'm in negotiations with a VOIP provider to install 130+ Yealink handsets across three buildings connected with fiber-optics. The provider will host the pbx (3CX I believe) and manage the phone system.

First they told me that I could not page outside of the POE switch the phone was attached to, then they changed their story to I could page any phone connected to any switch inside the building, but cannot page outside of the building.

They are telling me though that this system is incapable of multicast paging across all three buildings. Nothing in the manuals indicate any sort of limitations other than all the phones need to be on the same subnet, but even that can be mitigated with VLANs.

They tell me I can multicast an entire building from any phone in that building, but I cannot page all three buildings at once. Another provider told me that what I want is possible, but not with a hosted solution, the PBX would have to be onsite. With the hosted solution they need to use a STUN server and a Session Border Controller that disables multicast capabilities across the entire system.

If I can intercom any phone in the system, doesn't it follow that I should be able to page all phones in the system? (I'm assuming dialing extensions between buildings works, but I also assumed a system-wide page would work, as it does with our 20year old Nortel system).

Thanks,
Frustrated IT Guy