Help "End Call" button erratic

On ICS, but otherwise bog standard.

An annoying occasional fault -

On trying to end a call, sometimes the "End Call" button on the screen does not work.
The phone has not hung, because the back button works OK and it will do other things, but the call is still connected.
The only way to disconnect is to turn the phone off.

Happened twice recently when using voicemail, but has also happened on normal calls.

Any ideas please?