> There is a grey area here of calls from businesses which the customer is signed up to
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).
Another moment of potential irritation: companies being bought out or losing their network supplier giving that third company a business relationship with you.
Which is why nowadays I never take calls if the prefix is from a country I don't have connections with, it is private, or doesn't map to any of my contacts.
If it is someone that I actually know, they will be aware how to actually reach me.
Companies aren't allowed to do cold calling in my country. Wiped out an entire industry and nobody wept.
Scams are not what I would define as "telemarketing" though.
It would be helpful to know your country in this thread.