SMS's are of two types. When triggering an SMS you must mention the type.
Promotional – Noncritical messages, such as marketing messages. Most Promotional SMS's are delivered but a few can be undelivered - usually because your messages have been blocked by the end user or by the telecom provider.
Transactional – Critical messages that support customer transactions, such as one-time passcodes (OTP). Transactional SMS's are almost always delivered
All countries and telecom providers within those countries place numerous restrictions, especially on Promotional SMS. For example, you cannot send Promotional SMS to Indian mobile numbers during 9PM - 9AM.
SMS Delivery Rate: AWS has a hard-cap of 20 SMS per second. This limit cannot be raised. So plan accordingly.
DO NOT ever make the mistake of sending Promotional SMS's under the Transactional route. Newbies always assume they can get away with this practice. Mobile phone users tend to have a very low tolerance for unsolicited SMS messages. Response rates for unsolicited SMS campaigns will almost always be low, and therefore the return on your investment will be poor.
REMEMBER: You are billed for SMS's even if they are undelivered.
Additionally, mobile phone carriers continuously audit bulk SMS senders. They throttle or block messages from numbers that they determine to be sending unsolicited messages. Once you get on the block list, it is impossible for you to get out.