Difficult to see how , even if never recontracting to get a better deal , a telephone only service can be £87 a month , but I can see how that can be quarterly charge , whats more someone who has simply paid for a telephone for years on end never renegotiating for a new deal is quite likely to be on a quarterly bill , as it was broadband that ushered in BT’s preference for monthly billing , before that many ( if not all ) telephone customers were billed every 3 months.
If by some chance it is that amount per month , if she qualifies via being in receipt of some benefits (pension credit for example ) then BT Home Essentials (the cheaper social tariff ) is what you should go for , if she doesn’t qualify then telephone only , with the no broadband (from any source not just BT ) should be around £17 a month but has to be done over the phone it’s not available to order ‘on-line’ which is understandable if the application is supposedly from a no broadband user
https://www.bt.com/broadband/home-essentials
