The only real thing that is required is that you make sure the line to the alarm is filtered.
There are two ways about this
1: Use crimping tools and make up your own RJ11 and BT plugs.
This can be made to length and pluing to a half-decent filter.
Shamless plug, but something like the Excelsus Z-blockers I have available in the for sale forum provide this.
2: Use a hardwired method into one of the NTe5 ADSL faceplate replacments.
You have two options for this, one more expesive than the other, but dependant on how much control over you want over your extention cabling.
The standard ADSL faceplate provide a RJ11 unflitered output, and a filtered BT plug output. In adition, you also have a crone punchdown block on the reverse which provide pre-filtered output.
The modified version of the faceplate, provides the same standard outputs as about. Though in this case, a second output on the rear of the plate provides unfiltered service.
You can create a dedicated ADSL faceplate within your house this way, rather than having to use the front of the master socket.
For an alarm, my preferance would be a hardwired method into the rear of a ADSL faceplate. Saves you massing about with crimps, and the only tool you will need is a crone tool, which you can geta plastic one for £2.