I have an Inter-tel 8620 phone for work. At times the buttons on the phone do not function. I know that sounds crazy but none of the buttons will work on the phone. This is usually accompanied by a high-pitch squealing sound. And at times the only way to disconnect a call is to unplug the network cable. The only way I have found to resolve this problem is to reset the phone my unplugging the network cable and letting the phone reset. Sometimes this works on the first try, sometimes it has to reset 2-3 times before the buttons function again.

Has anyone else had this problem? Any ideas on how to resolve this problem?