In the United States, it is industry mandated to give messages receivers the ability to opt into receiving SMS messages for your 10DLC campaigns.
When registering your Campaign, you need to provide a START message. This is the message that a customer will receive when they reply START to the 10DLC linked to a Campaign.
An example of a START message is:
"Thank you for subscribing to receive SMS promotions, updates and alerts. Reply STOP to opt out."
An example of when to include START in a SMS message:
"You have been unsubscribed from receiving SMS promotions, updates and alerts. Reply START to opt back in to receive these messages."
START message best practices:
1. Use the name of your company and product in the message.
2. You must include how a message receiver can opt out.
3. Add contact information.
4. Disclose the frequency of SMS messages (if applicable).
5. Use a consistent message when possible.