We can forward the phone number of recipients that have opted IN or OUT of messages. Please open a support ticket and provide the email address where you would like us to notify when this happens. We use a similar setup as What happens if someone responds to a message (SMS)?
TO OPT OUT
If they respond with STOP, UNSUBSCRIBE, STOPALL, UNSUBSCRIBE, CANCEL, END, or QUIT (more details here), they will be automatically removed from all future messages until they opt in.
TO OPT IN
After opting OUT, they can opt IN by responding with START, YES and UNSTOP.