If you were going to block all her calls via a recorded message, what's the point in her having a phone at all, or would the handsets just be left as a decoy so she thinks she still has a telephone? You could just turn the ringers off her telephones but of course this means she won't pick up any calls, though she can still ring out.
Call Protect sounds good, if you switch her back to BT as a provider. Maybe you can get calls diverted by her current provider, rather than buying any new kit?
To answer you question about scammers though, scammers don't press the hutton to be connected because they don't usually hear the TrueCall message (see my previous explanation about how call centres operate. They usually only connect a phone monkey to the call when it has been connected your end). If a UK-based phone monkey does hear the TrueCall message and persists with connection, they are breaking the law and they will know this. You might think they don't care, but their managers do. It's much easier and safer for them to avoid numbers where someone is obviously scrutinising them and may record the call, which you have the option to set up with TrueCall.
I have monitored TrueCall online for 5 years and there have only been 2 occasions when a UK-based cold caller persisted to be connected, out of hundreds per year who were blocked. These exceptions were where someone felt they already had permission to call and were trying to speak to my mum personally, rather than just doing random phone trawling. In the case of the company whose DD you cancelled, they might be trying to get the payment reinstated as per their previous contract with your mum. So yes, these particular people might call her again.
The trouble with introducing any new telephone kit is that it likely to get dismantled. Cordless phones lose their batteries and battery covers. Handsets get left off the charger and with the line still connected to the last caller. Handsets are mistaken for the TV remote and vice versa. Handsets get put in the fridge, or the back garden or the dustbin (which is the preferred location for all 'broken' gadgets, i.e. gadgets that are not understood and therefore make the PWD feel threatened, confused, upset and fearful).
If you can find a solution with your tethered old fashioned phones it would be preferable. Having said that, MIL cut the phone cable with a pair of scissors because she wanted to move the phone to a different part of the living room. All concept of the purpose of cables had been lost and I partly blame us, her family, for introducing cordless gadgets.
