The defacto monitoring standard for Fire Alarm monitoring is now BT Redcare. This service is provided by an accredited Alarm Receiving Centre and allows notification of an alarm event but also more importantly when the line is no longer available. Redcare will 'PING' the line every few seconds to ensure that it is available. If not then an alarm condition will be generated.
Please note you do not need to contact BT for this service. King Communications will provide this as part of the installation and will test. There is however a setup and annual monitoring fee.