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Even normal phone numbers have a (low) price that varies by country and can change over time, so what would the default be?
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.