It's £12.20 if you have no broadband access in the house and pay by Direct Debit, this doesn't include a calling plan or any inclusive calls.
Generally most customers with no access to the internet use the landline more than your average household, if your mother uses the phone a lot then another option is Home Phone Saver which includes unlimited calls to UK landlines for around £22 per month, it comes with a price guarantee to avoid annual increases, along with a few handy calling features included.
More info about Home Phone Saver can be found at Learn about Home Phone Saver Plan
Another option depending if she meets the criteria is the new Home Essentials which although includes unlimited broadband which might not be required, it's only £15 per month with line rental included plus 700 minutes of calls each month to Uk landlines and UK mobiles.
Also again depending on your mothers needs, have a read of other support BT offer at Here For You
Ring 0800 800 150 and speak with the Loyalty team, ask for an account review to determine the best package for her needs, it might also be worth adding yourself as an account manager to manage her BT account for her.
I hope that helps 🙂
