I think you are right about this. The previous response said they received 10 spam calls for every 1 real one. I'm probably about the same. My email spam isn't even close to that. 50 to 1, inbox, maybe?
This situation is far more dire for the telco's and their debt holders than they are catching on to. The telephone number, portable or not, is what anchors their customers. When it is gone, you've got a commodity.
What we need is an abuse prevention system involving authentication. You can get reported for robo calls, and if enough different accounts you called have reported you, you get reviewed/banned/throttled. Let's not reinvent the wheel, it's been the standard way to deal with spam all over the web. But it requires rethinking the way we use phones, and a whole new infrastructure. It also brings some anonymity concerns but half-measures are just an arms race.