Dedicated Voicemail server

Hey folks, I have several sites that, due to bandwidth and link stability reasons, I have configured on stand alone servers. Two sites are running asterisk, while a third is running switchvox. I am using polycom 331 and snom330 phones.

The problem is that, while people can reach each other by dialing each other’s extensions, they can’t transfer voicemail between each other. Has anyone solved that particular problem? I had the thought of setting up a dedicated voicemail server, but I am unsure as to how to enable MWI on the phones for such a configuration. Does anyone have any recommendations? Has this problem been solved before?