I expect the Mods will pick this up to assist you .
Is there any form of security/medical system in your grandmothers house as it could well be this dialling out as part of its reporting function, via the broadband but registering as a phone call ( I think there are systems that do this).
The moderators of the forum have been informed of your problem. Once they have read this they may be able to help. They are a BT UK based team and if they can help they will reply via this thread asking you to contact them via a link. Once you have replied to them by the link, it can at present take up to three working days for them to re-contact you.
Please use the 'contact the mods' link in my forum profile to send in your Grandmother's BT account info and full details of the problem. You can find the link by clicking on my username.