This article will show you how to remove a Phone (SMS) Number associated with a subscriber to your site. You can only remove the SMS numbers of an Inactive Subscriber.
Who can use this feature?
System Administrator | Owner
Instructions
Navigate to Modules and on the Site Tools tab, select Subscriber Management:

Click Remove SMS Number at the top of the page:

Enter the phone number to remove:

Click Remove SMS Number:

View the pop-up that says "The number has been removed from the system"