https://www.acma.gov.au/telemarketing-and-e-marketing they give out million+ dollar fines which sort things out. Just being a customer doesn't allow them to send marketing, you have to opt in.
If they're calling from within the EU then a cross-country judgement is still enforceable, so I assume they aren't doing that.
Many of these calls are coming out of the Third World countries which are barely regulated in this regard, so good luck with enforcing it.
I've been getting a barage of non-stop harassment via unwanted magazine subscriptions. People Inc claims some company called "Subco" signed me up and won't allow me to cancel, only "Subco" can do that. "Subco" then claims they got my information from someone else down the line. They then blame someone else, someone else...
... and I still have no clue who the hell decided to sign me up for 20 magazines in the mail.
Pretty much exactly how it is in Switzerland. I once had a rather unpleasant experience with my old internet provider (Sunrise) where I asked the telemarketer who called me not to do so again, and was told they "had a right to do so, because I was a customer". I immediately cancelled, quoting this as the reason why (Sunrise would later try to tell me this never happened, but that's another story).
With that said, I think it does make a difference: if some random company calls, they can't lose my business because they don't have it; if a company I am a customer with calls and refuses to stop when I ask them to, they risk losing my business. I don't know if this is the reason why, but I barely get any telemarketing calls (never heard of anyone else here who has either).
If it is someone that I actually know, they will be aware how to actually reach me.
Scams are not what I would define as "telemarketing" though.