This is most likely a problem with the Hookswitch. After deploying Cisco IP Phones for the last few years we (and many others) have found this to be the most common hardware problem. There is a "Best Practice" for cleaning the Hookswitch contacts (link below) that does work on occasion. Usually the phone has to be replaced/RMA'd
Best Practices for Cleaning and Troubleshooting Cisco IP Phone Hookswitch
There was also a problem with the 7.2 phone firmware that caused similar looking symptoms but I am pretty sure that it was for the 3rd generation phones 7970/7961/7941. This could be rectified in the short term by booting the phone.
I'm not able to access my old voice mail messages all of a sudden. The recording says something like 'the message is currently not available'. This has never happened before in all the years I have been using this system. I have t...