Hi Ken,
Just to add a note to Tom's excellent post. This may be an actual problem with the Hookswitch but unlikely if a hard reset clears it. Just thought I would throw this in as the "Hookswitch" problem has been our most common failure for Cisco phones;
Checking the handset cradle clip.
Cisco IP Phones are designed with a reversible cradle clip in the handset. This clip is used with the plastic tab protruding out when the phone is in a vertical (wall-mounted) position. The position of the tab can interfere with the handset as it is placed in the cradle. If the phone remains in the on-hook position you may experience continued ringing when you try to answer a call or a lack of dial tone when you try to place a call. To resolve this problem, you may need to reverse the clip.
If the handset cradle clip is in the wall-mounted position and your phone is placed on a desktop, slide the clip upward to remove it. Rotate the clip 180 degrees and slide it back in so that the tab is hidden.
If the hook switch remains in the down position, tapping on the phone should free up the switch. You can also try pushing and suddenly releasing the off-hook button after the handset has been picked up.
Checking the hook switch contacts.
The hook switch contacts on the phone use a wiping action to self-clean the contacts. If your phone is not used regularly, dust and other airborne contaminants may degrade the contact performance and cause problems with operation. If you have periods of limited phone usage, you can clean the contacts by quickly pressing and releasing the hook switch a dozen times
From this doc;
http://www.cisco.com/en/US/products/hw/phones/ps379/products_administration_guide_chapter09186a00801ebe73.html#wp1097559
It sounds like you might be running into this bug for 7900 phones:
Release Notes
Symptom:
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.
Conditions:
This has been seen in various Skinny phone loads up to and including 7.0(2)SR1. It is not dependant on CallManager version.
Workaround:
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.
CSCsc99161
Phone does not go offhook except via softkey.
http://www.cisco.com/pcgi-bin/Support/Bugtool/onebug.pl?bugid=CSCsc99161
Hope this helps!
Rob
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