>>Can these security systems produce rein on the adsl line?
A.----Earlier models used wire wound relays, these would. Yet you say that you've had it for 5 years, so its very unlikely.
It must be solid state. One way to sort it, listen to the unit when you activate the unit, it should have a test facility. Don't forget to unplug it from the line first. Dont test it if you are connected to central monitoring.
Let me know the manufacteurer/model if you can, PM it if you like.
MC
Edit :- I'm guessing its not a wireless alarm is it?