Twilio requires that you call from either a number you have in their system, or a number you've verified ownership of. That's a higher bar than a lot of voip providers -- i've gotten many calls from numbers that don't even fit in the dialing plan.
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)
Telecom makes most "web scale" stuff look very small. Even a one-man operation might be doing 4000 calls a sec, each requiring about 20 packets, plus routing and billing lookups and records.