I would honestly NOT recommend the 3com 4500 PWR series of POE switches. I had the same problem crop up on more than one switch, even after having it replaced. In a nutshell, the switch would reboot itself without any sort of errors being thrown in the syslog. Replaced it with a HP Procurve 2650 PWR and haven't had a hiccup since.

We are using the 5x00 switch with Mitel 5212 and 5330 phones.