This is an unusual scenario which caused due new profile creation.
A user already have a phone number saved in the dashboard, is unsubscribed for receiving sms.
IF he choose to remove the number to use it for a new sign up via SMS by sending START as per the business configuration. This creates a new profile for the user with "Guest user" and the phone number.
Once a new profile is created, the user continued with the process and sent out the YES message for the sms subscription. As a next step, removed the phone number from new profile and add it back to the existing one.
Resulting to this both the accounts will be merged into the existing account with a sms tag on profile and a connected via SMS entry on the Guest timeline. 
As the existing account was not yet subscribed to the SMS notification the option still shown disabled.
Sending a "YES" again would change the SMS subscription for the user profile.
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