The TPS is effectively useless. It is a paper tiger. British based firms routinely ignore it and foreign firms completely ignore it. If you complain, very little or nothing will be done.
I have been plagued to such an extent this last 2 years, that my phone is now permanently "off the hook" If I need to make a call, I remember to take it back off the hook again afterwards. People I know call or text me on my Mobile, although I am now starting to have unwanted calls on that as well !
Taking the phone off the hook may not be ideal for the 93 year old lady, but it may be the only solution. Investigate a simple Mobile phone that she can use and pass its number on to everyone that needs to know it, and nobody else.
Caller display may be a solution but it won't stop the ruddy phone ring late in the evening, or on a Sunday afternoon. So the disturbance will still happen.
This is a modern plague and I can't see any end in sight.