That's one of their recommendations. So the idea of Twilio is just drop a library and we take care of telephony for you, but then they suggest you implement coo

That's one of their recommendations. So the idea of Twilio is just drop a library and we take care of telephony for you, but then they suggest you implement cooldown periods, exponential backoffs, use their other services (paid) to check if the number is a mobile etc. And they tell you this, after you get the nasty bill.