Every month I get a phonecall from an old dear.
It seems that she checks her phonebill every month and sees my number on the
bill.
She has no idea why my number shows on her bill so she calls me to
see why
my phone number is showing on her bill.
My number shows because she calls me every month after she has
looked at her
phone bill.
Any solutions?

I've had something similar with an old dear recently - we've got
caller display for missed calls in work, and I returned the call. She
reckons she didn't call me, but after speaking to her it transpires
she'd had problems with her billing ever since a new neighbour had
moved in (she's on a cable / virgin media phone line)

I suggested she got back in touch with them, gave her my number, and
I kept a record of the call time (early hours of the morning IIRC) in
case they wanted to argue the case.

Mistakes do happen, and it might make her feel better if you suggest
something similar ?