I have 10 remote PBX’s connected to my PBX here over T-1’s and I have one centralized voice mail system here. It works great.
I use Cisco routers. The main obstacle for me was to turn on the lights (message notification) on the remote phones, but it works now. I used a POTS connection over the existing T-1’s between the Cisco’s instead of a VoIP on my Cisco’s to keep dedicated channels on the T-1’s for voice mail lights. This allows the PBX’s to talk freely without setting up a call. I eliminated the need for a PRI just to signal between PBX’s, and for the long distance charges associated with it. Email me at firstname.lastname@example.org with more detail on your situation and maybe I can help you. Good Luck!