The same applies to the large scale phone fraud though, which quickly gets into tens to hundreds of millions of euros per year. Police won't do much because the criminals are outside Germany and cooperating with e.g. Turkey is hard, and with India it's impossible. Phone companies don't care because they're making money and the regulator is asleep at the wheel.
Let's just make the phone companies liable for damages where they can't produce the customer (no KYC) or the customer is international. They'll quickly figure out who is legit and who isn't when it actually hurts their bottom line.
The land line calls were mostly spam, mostly from established companies, and some scams. A side effect: they went from 70€/month to 15€, 22€ after the first year.
I had got them a 35€ contract (land/fiber + 2 cellphones) three or four years ago, but somehow Movistar managed to creep the bill to double and had announced that it will start charging 120€ this year. Because reasons.
The cost cutting can be immense as you mentioned. Plenty of people have ancient contracts that they never changed.
This would result in prepaid sim cards going away which would create a lot of problems for homeless people and illegal immigrants.