There are lots of reasons why a text message may bounce when sent to a user. Reasons can include:
Message is blocked by carrier
Message content is blocked by carrier
Destination number is not equipped to receive a message
Carrier suspended the destination number
End user is out of prepay credits
When a phone number doesn't receive a message because of a "hard" bounce, Tatango will immediately and automatically unsubscribe that phone number from your list. If it's only a "soft" bounce, that phone number will remain subscribed to your list until there have been 7 consecutive soft bounces.