Spam calls are actually a cash cow. Uses your minutes and Verizon also makes money selling a subscription spam blocking service that would be worthless without the annoying calls:
The receiving provider usually gets paid to terminate the call.
It may only be a fraction of a cent per minute, but these are the same telecoms that will gladly send you a bill every month because you owe them 1 cent.
In the past, cellular network operators earned revenue based off minutes sold. That's why voicemail has very verbose instructions that are read to you every single time. It's a way of inflating average call duration and thus revenue.
These days with unmetered calling, the receiving telco still makes some money off incoming calls through termination charges.
Seems like they would like to unclog their Network not to mention their servers have to handle and store all the voicemails.