Good morning.
An authorization letter can be used. Sign it and send with your DL or other ID.
If the number is not reassigned you should be able to get it back. Another way of validating it would be to provide frequent numbers dialed or last few top-ups that you can recall.
Go into the store or send your sister. The persons you call may not have full access (or know-how) to the areas that can give the information you need. Chalk it up to inadequate training and/or inadequate access based on support level.
Knowing the solution doesn't mean knowing the method. Yet answering correctly and regurgitation are considered "learning" and "knowledge".