Have a look at this bug, it sure sounds like the exact problem;
It sounds like you might be running into this bug for 7900 phones (first reported in 7.x loads but looks like it has carried forward to certain 8.x loads):
CSCsc99161 Bug Details
Rarely a 7900 IP phone will get into a state where the speaker, headset, handset, or line buttons will not work to take the phone offhook. In this state the only way to go offhook will be to use the softkeys.
This has been seen in various Skinny phone loads up to and including 7.0(2)SR1. It is not dependant on CallManager version.
A simple IP phone reset will recover the phone.
Further Problem Description:
The root cause of this issue is not known at this time.
This problem will be further addressed in the 8.0 Skinny loads by adding more debugging to the IP phone console. If you suspect you have an IP phone in this state running an 8.0 load please do not reset the phone and contact TAC immediately.
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