We have Cisco switches with class 2 PoE (7 watts per channel), Nortel Mediatrix Survivable Media Gateways and a large Nortel main switch. Handsets are variable type, some basic, some with login and large screens, but all are BT own brand. The whole thing is managed by BT. There are PSTN links into each gateway in case of network failure (multiple site).

One thing I found interesting is what class of power the handsets need, most seem class 2, but a few are class 3 and that has a knock on effect on what you pay for switches. We don't have any power bricks/injectors with our handsets, all done over the line.