The reason most services require a phone number is so you can't just create a new account if you get banned and ideally your account is somewhat tied to a real person. They ban VOIP numbers because it would defeat the whole point.

Part of SMS verification is to raise the cost of Sybil attacks. Well known sources of cheap bulk phone numbers are often banned for that reason.

I'm not sure if Twilio blocks sending to their own numbers but you can't receive from or send to short codes, which will limit you a lot when verifying with services like Uber.

Twitter SMS validation doesn’t even work with my regular phone!