How do you want your customers to recieve the number without internet?
SMS? Papermail? You could also email a file to their PC and then they copy&paste the file to a specific folder on the phone. Or you tell them the number and they write it into an edittext....
You can use Email from the Phone library to send the id from the device app.
Reply with a string that is derived from the key and let the user copy and paste it in your app.