A number that is removed from an opt-in list via AlertManager or the customer portal isn't actually opted-out - it is simply removed from the opt-in list. There is a subtle but distinct difference.
A true opt-out can only be performed by the customer using their mobile device. As a result, if someone is removed from an opt-in list via AlertManager or the internal portal it is not opted out and could be added to the opt-in list by uploading a file with that number in it or adding that number back in via AlertManager.
If someone opts-out from their phone they are truly opted out and cannot be opted back in using any method other than them opting back in themselves from their phone.
It follows that if a number is opted out from a phone it cannot be added back to the opt-in list via AlertManager. If you attempt to do that you will see an error message appear below the "Import Numbers" box on the List Maintenance tab.