I have a customer with a SV8100 with about 200 IP phones on it and there are 2 phones that continuously freeze up...the display is fine and phone looks as if its working but there is no dial tone and it can be called and rings once and goes to vm...if the phone is unplugged and plugged back in it works for a little while again.
The phone itself has been replaced,the jack and cable have been replaced,it has been moved to a new port on the POE and also delted completely and rebuilt.
Still these two phones lock up constantly.
Anyone have any ideas??

Have you ruled out duplicate IP addresses on the LAN?

I would also check and see if your POE switch is at max wattage as well as scanning that cable. Have you tried known working phones on those cables?

Try a NON POE port and a local power supply, and also as said before, make sure an IP address is not duplicated.

It would be also good to know what phone models are we talking about. In any case, I had the same problem on SV8500, but only under specific circumstances: when a softphone called an IP phone. The problem there was an application on the computer (HP Thinclient) that had the softphone running on it. Once the app got removed, the problem got solved.
Before that, we also looked into firewall settings between our different VLAN-s, but those didn't lead to solutions.
Just throwing ideas at you...