Recipient wireless carriers always reserve the right to filter out incoming SMS and MMS messages (aka, carrier filtering) from certain numbers, and routinely do so to protect their users from spam. For more information on carrier filtering, as well as tips for checking and preventing this, please see our article How Does Carrier Filtering Work?.
Bulk Messaging Campaigns
If you'll be sending SMS messages as part of a bulk campaign (with 30 or more identical or similar messages going out within a few minutes), we discourage you from using regular 10-digit numbers. Instead, we highly recommend using an SMS short code, which has higher messaging throughput. For more information, please see our article What is a Messaging Short Code.