If you want to delete mobile numbers that have previously failed to send, you can do this by locating them within your address groups (if you imported the number into Message Box) by using the 'Find field' in your Manage contacts area, once found you can then delete the number by clicking the 'Delete' button.
If there are a large number of failed messages, you can simply download the delivery report to an Excel spreadsheet.
To learn how to do this click on the link below:
From here, you can then filter out the failed numbers and delete these. You can then re-import this new set of numbers back into your contacts area.