If they spoof the caller ID, then they are probably not "telemarketing" but some kind of scam calls, like those pretending to be from a Microsoft support and want to install a trojan on your computer and get your online banking details.
Yes there is a difference between telemarketing and scam calls. If I get a proper telemarketing call, I usually say quite early in the call, kindly but firmly, that sorry I am not interested, and hang up the call. Not making any stupid excuses like that I am quite busy right now etc. because then they just say they will call back in 30 minutes when I am not as busy.
It is still their ("legit") work so I am not going to behave like a dick towards them. In a way I pity them that they have such a shitty job, I would rather sell my ass than my voice in telemarketing. Pays better too.
If it seems to be a scam call, then I don't say anything and just hang up. Better luck next time, buddy.
Considering you called the caller an "a-hole", I am not surprised they reacted negatively to you too. Not saying you shouldn't have used the a-word if it indeed was a scam call, but still understandable from the other side. He is just trying to feed his kids that he doesn't even have!
Not sure if it is an European-wide thing, but at least here you can tell the operators that you don't want telemarketing calls, after which the legit telemarketing calls will stop. But the scam calls won't because naturally they don't care of such restrictions.
The legit telemarketing calls can be quite sleazy sometimes. A relative a mine, who is too nice to say "no" firmly to telemarketers, bought a tablet and a data SIM subscription for it, with a monthly payment. She got the wrong impression that she'd get the tablet for free, which wasn't the case.
I got a similar call from the same phone operator and I also got an impression at first that I would get a free Android tablet if I continued my current mobile phone subscription with them, but I noticed the monthly payment would be higher than what I have now so I asked why was that, and then it became clear the point was that on top of my current subscription, I would order an additional data SIM and the Android tablet for an additional monthly payment.
Quite sleazy and I almost fell for it, luckily I was awake and realized it was not just the continuation of the current mobile SIM subscription but an additional "service" and fee on top of it.