From what I remember, you can change in and out of phone numbers like with Google voice - and there are a ton of them. I don't know if that matters for spammy calls - I don't know anything about that domain.
Twilio allows you to automate calls programmatically, and what a lot of the callers do is wait for you to say hello - they're robo dialers. I wonder how much Twilio has influenced that (unknowingly or deliberately)