"It also gives the security system free access to the line..."

Just for the sake of clarity, that's "free" as in "unimpeded" - not "free" as in "without cost".
You will still have to pay your standard tariff (whatever that may be) for the call!

Of course, if the intruder has the foresight to disable your phone line before your dialler (eg, by cutting or bridging the line outside your house), you are stuffed.
The only way to protect against this is to install monitoring equipment in the telephone exchange.

Or perhaps you should consider a GSM (or similar) backup?