If a recipient of an SMS message sent via Phoneburner replies with one of the following words in reply to an SMS message, they will no longer be able to be sent SMS from Phoneburner.
STOP, STOPALL, UNSUBSCRIBE, CANCEL, END, or QUIT
They can re-subscribe or ask to be added back after unsubscribing by sending a reply back:
START, YES or UNSTOP
At this time, there is no flag on the Phoneburner side to indicate if a use has unsubscribed from SMS other than the words in the messages appearing in the SMS tab.