I think you are going about this the wrong way- you keep asking about a separate alarm/sensor system to page you...

when all you want is a circuit that pages you when the factory alarm is set off! No new sensors, no new mpotion detectors- simply you want a page when the factory alarm and factory sensors are tripped.

Here is something that you just connect the BMW "alarm out" wire to, add a SIM, and it will call you anywhere.

GSM cellular Auto Dialer, Dakota Alert CAD-05-Kit


most "pagers" are very limited in terms of distance- they just page the little key fob.