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Why is this not set to zero dollars be default?!?

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.

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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 wonder if this is why I don't get SMS OTP from some apps when I'm abroad and roaming...

AFAIK the sender shouldn't have to pay more just because you are roaming. That's between you and your provider.

I would imagine there are rules/regulations about a SMS provider blocking communications before fraudulent behavior is determined? Not saying it shouldn't/couldn't be done, but probably one of those things with a simple tech fix but a complicating social/business aspect.

It could be an option in the API call with a default in account settings. I bet most people who are trying to reduce spam accounts by requiring a phone number would actually prefer to exclude these numbers anyways.

surely not if the customer _explicitly requests_ that the communications are blocked? iirc in Aus it was possible to have your provider block messages to premium rate numbers back in the days when it was popular to buy ringtones.