Well over 90% of alarms report to the monitoring station over your regular telephone line using a digital dialer. When your alarm goes off, it uses your telephone line to call the station and relay specific information. By installing a CA38A Jack, the alarm is granted line seizure. This means that if anyone was talking on the telephone, or if an intruder took the telephone off the hook, it would not matter. The alarm would seize the line and call out to the monitoring station. If somebody were to cut the telephone line, your alarm would still work, but it would not be able to communicate (report) to the monitoring station. People should physically protect exposed areas of their telephone lines with conduit. You can also add line protectors to your CPU.
They are inexpensive. If somebody were to cut the telephone line, the alarm immediately goes off. This means that the intruder would still be standing on the outside while your siren is screaming. In this scenario they are less likely to continue the break in.
As an option, there are cellular back up and/or primary systems available should the telephone lines be out of commission or not available for any reason. There are additional hardware and line charges with this system but they do ensure that an intruder cannot disable your alarm by cutting the phone line.
For business' or residents who desire a higher level of security, a dedicated circuit is used to communicate with the monitoring station. Although more expensive on a monthly basis (additional Bell line charges), it is still affordable security, If the communication link between the premise and the monitoring station is broken for any reason, the station receives a signal immediately and dispatches the proper authorities.