Longarm,
I have adsl and a monitored alarm on the same line, no problem - but it depends what type of monitored alarm you have.
If it is 'Redcare' or similar then the two are NOT compatible. The Redcare alarms (normally only used in high risk areas such as post offices, banks etc) are constantly in touch with the centre via an 'open' line. Similar to ADSL! If the alarm stops responding, or the line is cut, then the centre is triggered to investigate. ADSL interferes with this capability, so the unit needs filters to work properly.
With a normal home alarm, the unit simply dials out via a modem when the alarm is triggered, so uses the 'normal' phone line signal. The ADSL does not interfere with it.
Unless you are paying a significant amount for the monitoring, then you have the second, compatible type of alarm.
Hope that helps.
WH