It was the case in France too, it was called Bloctel; you just had to register your phone number and telemarketing companies had a legal obligation to look it up and not call you.

So it worked for a time, legit companies stoped calling; however most people didn't knew about Bloctel as it was little advertised.

Meanwhile, spam calls turned into scam calls, and a real issue especially with the elderly.

So I think that banning all unsolicited calling kills two birds with one stone, spam becomes illegal and scams as a consequence becomes easier to spot as there are no more unsolicited calls.