Twilio allows 1 message per second. Plivo allows 10. If you use a naive for loop, you will hit 429 Too Many Requests in three seconds.
It automatically paced the sends to avoid being flagged as spam by carriers. Putting it to Work bulk+smssender+github+work
Most automated systems will block you if you don't provide a "STOP" option. Twilio allows 1 message per second