I suspect your relative has misunderstood, if they have been migrated to Digital Voice, so the ‘phone’ is now to be connected to the BT router , then that’s a done deal , ordering a new line will achieve nothing , as that will be provided on the same basis , as a DV phone service .
Alarm providers have had decades to prepare for this , if the opportunity to delay DV ( by indicating the presence of an incompatible alarm ) was missed and the conversion to DV has happened, then I doubt there is any way to go back to the PSTN ( legacy ) phone network….FWIW , if your relative was a ‘new’ customer of this alarm company, ( so a new occupant of the ‘development’ needing an ‘alarm’ , the alarm provider would have to provide a newer version of whatever alarm they provide that can work alongside DV , so really all the alarm company needs to do is upgrade your relative to their current alarm ( as they will do with brand new customers ) rather than fob off their existing customers base with the old alarm equipment , saying to them , delay your conversion to DV for as long as possible …( because it suits the alarm provider ) this is really an issue for the alarm provider
