Ниже приводится цитата из длинных кодов sms awssupport документации.
In the United States and Canada, sending rates for long codes are restricted to 1 message per second. This limit might be higher or lower in other countries and regions. If you send a large volume of messages from a long code, wireless carriers might begin to block your messages. If you send SMS messages programmatically, your applications should limit the number of messages that they send per second.
Надеюсь, это поможет.