We have 7 SPA504G phones recently deployed, connected to an ESW 520 24P PoE switch.
At very random times, a phone will "freeze" such that the screensaver is stuck and the time displayed on it stands still. Pressing keys on the phone including menu button do not show any results on the screen of the phone.
Oddly, I was able to ping the phone when this happened and also load the web interface in my browser. The web interface shows the current time on the phone being the same time that is "frozen" on the screensaver.
This doesn't seem to be related to a calling or QOS issue as my phone froze at 6:18am before anyone was in the office.
All phones are running the current latest firmware, 7.4.6