Spam calls aren’t made like normal calls are. I got a voice spam call on my new phone before I connected it to a carrier. In other words, it didn’t even have a phone number and I still got a spam call on it. So however it is connecting, it’s not doing it through normal channels.
Another problem is that most of the spam calls I get don’t even reply if I do pick up the phone. I don’t know what they get out of it but I can’t respond in any way much less offer to talk to them for a fee.
I’m not knocking your efforts to come up with a solution, conservatism_IS_compassion, but I don’t think that one will do it.