Abstract: In a method of monitoring the operation of a subscriber unit in a telecommunication system, in order to detect possible malfunctions as early as possible, a call reserved in signaling for some other purpose is transmitted to the subscriber unit, whereupon a call to which the subscriber unit automatically responds is selected as the call. A message about receiving the call is awaited from the subscriber unit, and, if not such message is received from the subscriber unit, the call is repeated until the subscriber unit has left a predetermined number of transmitted calls unanswered, whereupon an alarm is given.