But how do you handle "call transfer" from one person's cell to another? And how do you distinguish between calls that go to Voicemail if unanswered vs. those that are Call Fowarded on Busy (or unanswered)?
voip.ms extensions must start with '10'. So you could have extension 101, 102, etc. If your existing extensions are 123, 124, etc. and you are unwilling to change them, then voip.ms will not work unless you put PBX software infront of it.