James
We've had this come up numerous times in fact I'm surprised that
bbb_uk didn't jump on this straight away. He's dealt with most of them previously so why don't you pick his brains.
As the alarm unit is only on a short time, you don't pay very much so it doesn't matter whether it's a Non Geo or not. You'll only pay the basic charge for it ie. the minimum (on BT). If you don't pay for your Geo calls (inclusive or whatever) I admit that you're paying more, but not much.
Most of these units are preprogrammed with the 0870 number so it's nigh on impossible to change it without having an engineer to assist you by -
i. Giving you a Geo alternative (like rocking horse wotsit)
ii. Helpng you to reprogram the eeprom with the alternative data
. . . and we all know how likely that is