A hard bounce for a mobile number happens when an SMS message fails to deliver. There are several reasons why this might happen:
The number you’re sending to isn't correct.
Mobile networks report that this number is no longer active.
A device does not allow delivery of messages from unknown numbers.
A device has an inbox storage limit which might be at capacity.
Some older mobile phone models still have this limitation.
If your SMS campaigns hard bounce three consecutive times to the same mobile number, we automatically suppress that mobile number to stop you from sending messages that cannot be received. It is possible to prevent this automatic suppression if you want to.
Turn off hard bounce automatic SMS suppressions
To turn off automatic mobile number suppressions after three hard bounces:
Expand the User menu and go to Settings > General > Account settings.
Under the heading Features, clear the Suppress SMS contacts that have hard bounced (recommended) check box.
To turn on, select the check box.
Select SAVE SETTINGS.