Insider One now offers a centralized and flexible way to manage multiple SMS unsubscription methods directly from your panel. You can customize opt-out settings per campaign and localize unsubscribe keywords. Refer to Centralized SMS Unsubscription Management for further information.
With the SMS Unsubscription with Keyword method, you can collect unsubscribe requests from your users via text messages that include a specific keyword.
To use this method, you need:
A phone number / Numeric Sender (Short Code, LN, VLN, or TFN)
Unsubscribe keywords to collect messages with opt-out keywords
It is not possible to collect unsubscribe requests using the Alphanumeric Sender ID.
After you decide on the Unsubscribe Keyword and Phone Number with the help of the Insider One team,
Navigate to the SMS campaign listing page and click Settings.
Click the Unsubscribe tab.
To create a new unsubscribe method, click the +Create Unsubscribe Method button.
Ensure the Unsubscribe toggle is enabled.
Type a name for the unsubscribe method and select "Keyword" as your Unsubscribe Method type. Click Next to continue.
Enter the Unsubscribe Keyword and Unsubscribe Number.
Click the +Add Unsubscribe Variables button to customize your Unsubscribe Sentence.
Add a Confirmation Message to inform your users.
After filling in the details, click the Create button.
The Unsubscribe Sentence is added to your messages by default.
If you prefer not to include an unsubscribe sentence in your SMS campaigns, disable the “Enable Unsubscribe Sentence” toggle during the Design step of the SMS campaign creation process. 
When you change the default unsubscribe method, it will automatically be updated across all existing journeys in Architect, regardless of their status. This means changing the default unsubscription method will also update all SMS elements using the current default across active journeys. Please proceed with caution, as this may affect ongoing communications.
A user can receive a maximum of five confirmation messages per day for the unsubscribe process.