Abstract: In a telecommunications system, the amount of time a subscriber device is effectively out of service as a result of some fault or failure in telecommunications equipment is monitored. Subscriber devices are connected by corresponding subscriber communication paths to receive information from or transmit information to a telecommunications center such as a local telephone exchange. One or more data records is maintained in computer memory for each subscriber path connected to the communications center. Each data record is updated using recently determined data relating to the operational status of each subscriber path. The computer then determines from the updated record in accordance with one or more subscriber line blocking criteria a subscriber path blocking parameter. The subscriber path blocking parameter may represent for example an average amount of time each subscriber path satisfies the one or more subscriber line blocking criteria.