Like the title says, we bought twenty 8851 phones about 2 years ago, and in the last 12 months, 4 of them have "died" - the phone will reboot mid-call, and when it comes back, none of the physical buttons work except for the receiver button (picking up the handset works).
These phones can receive a call, in that picking up the handset works, and they can hang up a call, but you can't push any other button. Not the circular navigation buttons, none of the number pad buttons, none of the programmable quick-dial/favorite buttons, nothing.
I've tried a factory reset, but the methods I've seen don't seem to work - After holding the # and plugging in the PoE, the green lights on the headset/speaker just flash once. None of the number combinations (123456789*0# or 369#) seem to cause it to factory reset.
I can log in to the phone web interface, and I've spent an hour looking at the log messages, but nothing sticks out to me. There is a 2.5-hour gap in logs: 2 hours before it rebooted and broke, to coming back online 30 minutes later. So anything that logged before/during the reset are gone.
I've also bought a power brick and tried that instead of using PoE, but that doesn't make a difference. We're using a generic Linksys PoE switch. The phone registers with the call manager, the red VM light will light up if there's a VM waiting. Everything looks normal, until you try to use the device.
I took broken phone #2 apart to see if there was some stuck button/contact that would cause it to not respond to any other button presses. I booted the phone w/o a shell and none of the graphite pads/rubber contacts on the motherboard, and still the blue ring buttons don't respond. in this state, touching the receiver button still puts it into dialing mode, but nothing can be done after that except to hang it back up.
I saw an older post with seemingly the same symptoms, but no resolution.
https://community.cisco.com/t5/ip-telephony-and-phones/ip-phone-8941-buttons-stop-working/m-p/2311883#M244906
