Between this thread and the one you posted on Sunday, you really have already received the best selection of options available to you.
As suggested on Sunday's post, the zero cost option would be to set up the different ringtones on your existing Gigaset cordless so that VIP (known) callers in the phone contact list get one ring tone and anyone else gets another. At least that way your grandparents will know by the ringing sound if it is a welcome call or not. It won't, however, prevent the phones from ringing with the unwanted calls. Unless your grandparents have hundreds of numbers logged in the phone that is fairly quick and simple to do and free with the phone you already own. On some of the newer Gigaset models I believe you can silence the phones altogether on a timed schedule and the only calls which will ring through will be those on the VIP list.
I have used the trueCall standalone unit for almost two months now and it has entirely killed off the problem for me (with a bit of fine tuning to the settings). Zero unwanted calls getting through (although I can see the unit processing spam calls without ringing my phone and have made a few random checks via 1471 to confirm them as spam afterwards).
The CPR call blocker products https://www.cprcallblocker.com/ have a range of price points (with presumably different range of functions). You could look into the spec's of those to see if they will suit. I have no experience of them myself.
BT also do a range of call-blocking phones with trueCall features but see below ref. possible issues in setup for those.
If you have signed up for the VM protection offerings (call rejection etc.) you will be paying £2 or £3 per month as a recurring cost for a product which only offers a minimal solution to the problem. After one year of paying the VM monthly fees, you will be well on your way to paying for one of the entry level call blocking devices.
If you do go ahead with one of the call blocking devices, keep in mind that there have been a few posts on here recently regarding issues of setup and reliability regarding how the devices read Caller ID and process the calls. Although the blocking devices are good, some people have found they are not 100% reliable on their own particular home setup/line. These issues seem to be few and far between but worth keeping in mind that, wherever you get any device from, make sure the supplier has a good returns policy in case you do meet any issues when you connect it up in case you need to send it back.
You can download the trueCall manual from here for free https://www.truecall.co.uk/category-s/116.htm Have a read through it to see what the unit can do and how it can respond to the different types of spam calls. You will see the scale of the problem is a lot bigger than the basic anonymous call rejection and TPS kind of response.
After putting up with the problem myself for 6 months, I can appreciate how annoying it is and especially so for older relatives. Hope you are able to work out some suitable resolution from the info. on the forum.