I just saw Microsoft (relavitely) new VoIP ad compaign called "VoIP As You Are". The ads on the MS site are cute, showing two old PBXs having a chat. Apparently you get a different ad every time you click.. so keep clicking ;-)
So what this implies is that Microsoft is taking into account that the bigger companies will find it hard to switch to VoIP if they have to ditch their old system and start a new page. It also means that old vulnerabilities in PBX servers will probably be exposed to the less friendly networks (such as the Internet).