Summary
There has been random issues with phone's not updating there firmware or getting stuck during the reload. Phone's then exhibit audio and calling issues.
Perform this factory reset to possibly fix your issue.
Body
To reset a Cisco IP Phone 8811 to its factory settings, you can do the following:
Unplug the phone's power adapter or LAN cable
Wait five seconds
Press and hold the # key while plugging the phone back in
Release the # key when the Headset and Speaker buttons are lit
Enter the key sequence 123456789*0#
Wait for the phone to complete the reset process and the main screen to appear