If you get a prompt like "This number has chosen to unsubscribe from messages," it means that you had texted STOP at some point in the past. Remind can't send messages to that number. We can't unblock the number, but you can! Just text a class code(like a class you are already in) to 81010. This will unblock the number so you can link it in your account or just start receiving messages again!
If you are trying to add your phone number to your main account, you may own classes in a separate account associated with that number. You can reach out to support with your number to see if this may be the case.