Under normal circumstances, I have Twilio set up to bill us $500 if the balance ever dips below $500. That $500 is our rebilling increment. Each SMS message and phone call we made caused us to get charged our rebilling increment. We hit $3,500 before our credit card company started rejecting charges. I think that if they hadn't, we would likely have saturated our credit line.
I'm thrilled with Twilio's response to this issue, and most other transient issues I've had being a Twilio customer. The system is mostly rock-solid reliable. I actually went to bed during the middle of this event (midnight, Japan time) because a) my systems were reporting that our messages were successfully going out (so no customer-visible downtime) and b) I had total confidence in Twilio to take care of things. And they did.