Monitoring Fire Panels via phone lines is a technology that originally replaced the older City Tie style monitoring. The idea of using analog phone lines for fire panels has been around for many years, but in a world where we are moving away from copper, the price of a copper phone line costs more and more, and POTS monitoring is starting to be pushed away.
Phone line monitoring is pretty interesting, and it uses two key components. There is the Universal Digital Alarm Communicator Transmitter (UDACT) and the Digital Alarm Communicator Receiver (DACR). The UDACT makes a call which is received by the DACR automatically. This signal works similar to a fax line, as it is data transmission over the phone lines. When the data is received by the DACR, the monitoring center will convert that into a readable format that will show up on a dispatchers computer, in which they can dispatch the fire department, and contact anyone on the monitoring account.
An extra benefit of monitoring systems in this way is a feature called Contact ID that most modern panels are capable of. This allows the monitoring center, and in some cases the local fire department, to know exactly which device activated. If the fire department is setup with the right software, they can even see a floor plan of a building and see exactly which rooms in that building have an active alarm, allowing them to follow the path of a fire before they even arrive on site.