From your dashboard, click on your profile:
Then select Settings:
Can clients respond to the phone number they receive the SMS notification from?
The phone number used to send SMS notifications is read-only, so can only be used to send notifications.
Agent unsubscribed now wants to receive SMS notifications again?
To re-enable SMS, the user needs to send "START" to +1 614-665-8461, and they should be resubscribed.