Just a quick note regarding SimpliPhones.
A week after I installed it I had to drive to the site on a Sunday to investigate complete system failure. The issue turned out to be a loose hard drive power cable, probably a delayed effect of transit.
I mentioned the system runs on Windows XP. Well, it is no more stable than your average Windows XP PC. Even optimistically this is not enough for a business phone system. The client decided to return it (through Costco) and went with a Toshiba PBX which has gone swimmingly, and for about the same cost, with more features and perfect stability.