To join an organization's members list from your phone, set a join keyword in your account control panel settings. Now send a message to your account number with this syntax: JOINKEYWORD. You may also add your name to set your name with this syntax: JOINKEYWORD Alex Smith.
Here is an example how your customers now join your subscribe list.
To: Your account phone number
Message: JOIN Alex Smith
The response will be:
From: Your account phone number
Message: You have subscribed to receive messages from YOUR ORGANIZATION. Reply with STOP to stop messages or START to resume