If the alarm is on the phone side of the filter why can't you use ADSL elsewhere and why don't you need filters elsewhere?
Normally, the alarm is permanently connected to the incoming line, and all other phones in the house are connected after the alarm system. In an alarm situation, the system can hang up any call in progress (by disconnecting all the other phones) so that it can dial out. Under normal operation, the alarm system will detect if the line is in use, and make periodic 'status calls' to report things like the condition of backup batteries. You can treat this like a 'normal phone system' but there are two potential problems with this; The alarm system won't recognise the dsl signal and will dial out over the top of it. Since it's not filtered this will disconnect your DSL connection. Having your connection dropped occasionally probably wouldn't bother most people. The alarm system won't cleanly disconnect the DSL signal and it's not filtered, so there's a chance that Telecom's end will continue to make 'DSL noises' and interfere when the alarm tries to dial out. Having your alarm system fail to report an alarm would probably bother almost everyone. -- Disclaimer: Any disclaimer attached to this message may be ignored.