[pilif]

I would assume from the article (I have no experience with Twilio) that there's some "purchase X more SMSes when I've fewer than Y SMSes left" feature. So whenever the customer uses their API to send an SMS (or make a call), the software would detect that there are fewer than a configured amount of messages left and would bill the customer to top off the account.

The actual billing would go through ok and the master database would likely get updated, but the frontend that's doing the sending won't see the updated balance, causing it to purchase more credit.