SWITCHING:
Old switches should be considered for an upgrade and HUBS MUST GO! One hub on your network and you will have all sorts of issues. Cheap 8 port (or less) hubs should be pulled too. Install new runs or look into higher end switches. I recomend the inteliJack series from procurve if you have to have a small switch. They are managed and can do pass through power if your main switch is POE+.

POWER:
POE is VITAL to a reliable VOIP solution. Many people forget that if the power goes out you want the phones to keep going for at least 30 min to an hour plus. By doing POE you can cenralize your UPS units to the closets and have your poe switches run off of that. Otherwise you need to have a UPS everywhere you have a phone.

Other than that when you decide on the system talk to the Manufacturer. They almost allways have a “basic requirement” list.

Cheers!