There are two basic ways to contact the security company when an alarm signal is received; the telephone or two way radio. The telephone system depends on the fact that the telephone wires are not comprised and if you use cable phone that there is no power failure and the modem is functioning (You can protect against power failure for cable by having a UPS).

The two way radio system is an independent of all phone systems and is by far the safest and best system you can get.