Note: Now that Workflows are live in all accounts, you can do everything that Triggers and Campaigns do (and more!), all in one builder! Click to learn more about Workflows.
SMS opt-in campaigns (or "text-in campaigns") are great for clients who want to rent billboards or send postcards with an offer that customers can claim by texting something to a Twilio number.
The key to SMS opt-in campaigns is your "Customer Replied" Trigger condition, which should look something like this: