Recipient carriers always reserve the right to filter out messages (aka, carrier filtering) from certain numbers, and routinely do so to protect their users from spam. For more information on carrier filtering, please read the article How Does Carrier Filtering Work?.
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. Learn more about what a short code is here.
To give some context, there are two types of phone numbers that can send and receive SMS: longcodes, which are regular 10-digit local numbers, and short codes, which have 5 or 6 digits. If you're using Twilio for bulk outbound messages, they must be from a short code, as US mobile carriers do not allow bulk outbound SMS messages (whether solicited or not) on longcodes.
Short codes have a fixed monthly rate and an application process to let mobile carriers confirm that your business is not sending spam, and that your app provides ways for users to stop receiving messages. After mobile carriers approve your short code application, you're able to send a large volume of approved SMS messages at once without running into recipient carriers' spam filters.
If you're interested in a dedicated US short code, you can learn about how to get one from Twilio at our short codes page. You'll find information on pricing and capabilities, as well as on how to get started with applying for a short code.