I agree with other comments here. $0 is the minimum amount people should be willing to pay if they're not disputing charges or reporting fraud to the credit card networks / regulators.
No idea. I think there's a real problem with the whole design of premium numbers because I'm not sure how one is even supposed to know when payment is required or meaningfully accept it, though at least the API apparently allows this.
FWIW, I do think $0 might make a sane default, but you do understand that the user would have to change it from $0 before they could use the account, right? The whole point of using Twilio to send an SMS is because you wanted the SMS to actually be sent, which means you are going to have to pay for the SMS, and SMS is always stupidly expensive.
I agree with other comments here. $0 is the minimum amount people should be willing to pay if they're not disputing charges or reporting fraud to the credit card networks / regulators.
Time is money, after all.