I've been trying to find out why a previously working extension suddenly says, "Not Valid" when the phone goes off hook and "ACCNT" appears briefly in the lower right corner of the LCD screen.

After pressing the account button I am querried for an account code which neither I nor anyone else knows what this could be. I've checked account code programming and cannot find how to remove this feature, if this the correct reason why the phone gives a fast busy tone and 'not valid' and 'accnt' messages when off hook.