Hi there
You could try a couple of things. Try to restore to factory defaults. Next plan is to upgrade the firmware on a couple of phones and test. Could be a case for TAC and find out any hardware problems and raise RMA.
PERFORM 7821 IP PHONE FACTORY RESET THROUGH PHONE KEYPAD
Step 1 Unplug the phone:
If using PoE, unplug the LAN cable.
If using the power cube, unplug the power cube.
Step 2 Wait 5 seconds.
Step 3 Press and hold # and plug the phone back in.
Step 4 When the light on the Mue button and handset light strip turns off and all other lights (Line button, Headset button, Speakerphone button, and Select button) stay green, press 123456789*0# in sequence.
When you press 1, the lights on the line buttons turn red. The light on the Select button flashes when a button is pressed.
After you press these buttons, the phone goes through the factory reset process.
If you press the buttons out of sequence, the phone powers on normally.
Warning –
Do not power down the phone until it completes the factory reset process, and the main screen appears.
Hope this helps!
Cheers
Rath!
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