I hate to sound like a conspiracy theorist, but it's pretty easy for a carrier to determine devices that are making spam calls because they log everything, and they could simply create and distribute apps to their customers to enable reporting of spam calls, but somehow for years they've left it up to dodgy 3rd party app providers and the calls keep rolling.

I don't think carriers have any incentive to stop spam calls because they gain a lot of money every year in billing minutes for those spam calls (mostly prepaid accounts are affected by the billing unfortunately)...

I wouldn't dare go as far to say that the calls are possibly even sponsored or conducted by profiteers in the game... (People who sell prepaid and metered phone services)

Just a personal opinion though.