Have you been hearing buzz about Microsoft Lync? Microsoft is spending millions on marketing it - and I hear that the Fortune 100 have deployed it but in limited roles.
My brother works for a Microsoft integrator who has 135K seats deployed. I'm not even sure what that means, because Lync isn't being used exclusively as a landline or PBX replacement. It has many uses and not all of them are apparent. For example, it is can be deployed just for Presence and IM/chat. It can also be used for a conference bridge (like in the Office 365 bundle - does that constitute a seat?).
Sure, it CAN be deployed as a voice replacement BUT you still have to have SIP trunking from a voice provider. (Lync is not a dialtone provider; that will come from the SIP Provider.) Lync will act like a PBX in this setting.
Remember that Lync is the 3rd edition of Microsoft's Office Communicator Server. IMO, MS has not decided what they want from it yet.
Skype, mobile apps, messenger, Presence, PBX, conferencing -- it is all very cludgy. By that I mean, it isn't straightforward; it isn't user friendly.
My fears lie in the fact that Microsoft can't make a product that doesn't have to be patched every day due to too much bloated code and too many unnecessary features. And Lync has a lot of features. (Adobe is giving it a run for its money in patching Flash though.) Then by the time the user has a stable operating system (like XP SP3), Microsoft rolls out a new one - and we start all over again (from unstable and what many would call beta!)
My brother likens Lync to Sharepoint. Once people know what it can do... Well, more like, once it is thrust upon the users.
When you try to be something to everyone, you end up lost.