Harry Bloomfield presented the following explanation :
snippo was thinking very hard :
Hi. I've got the above system in my house. It went off accidentaly the
other day and someone rang me to say they had been informed that my
alarm was activated. Unfortunately I have no idea who it was and forgot
to take their number. I think they were associated with a previous
owner. Is there any way to reset this number to ring my mobile if the
alarm is activated again?
Cheers john

There is a manual for the system at

http://www.expert-security.com/manuals/700l.pdf

but no mention of an autodialler or the settings of it.

Try pulling the backup, it should then go back to defaults:-

User/Customer Code 1: 1234
User/Customer Code 2: 0000
Engineer Code: 7890

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk