Most homes in North America have phone lines that come into the home via over head wires, the wires run down the home and terminate in a plastic box then connect to your alarm system and your telephone jacks in the house. The wires can easily be cut and the box easily accessed. Any burglar can do this very easily and can disable your phone line which will then disable your alarm system monitoring. If you have your phone lines exposed on the side of your home you should always install an alarm system cell backup system which will provide your alarm system monitoring via the cell network.
When you purchase a new security system or hookup monitoring to your existing system the security system company your dealing with should be informing you about this as part of their security recommendation. Some inform the customer about this vulnerability and many do not as all they are interested in is getting the sale and adding additional cost makes a harder sale. Depending upon which security system you have installed there are a few options for cell backup, it also depends upon the company your using for monitoring as companies will use different systems.
If you have a DSC system your alarm company would probably install the DSC cell backup, if you have the Honeywell security system they would install the Honeywell cell backup. Both are very good systems and provide monitoring with full communication if your phone line is not working or has been cut. These cell backups also have their own alarm system battery for when the power is out.
A cell backup usually costs between $200 and $400 upfront and aprox $15 to $20 per month for the additional monitoring fees.