It depends on how this system became 7.1
If its an upgrade from an earlier release there may not be any PAK code for the phones or nodes. There will be a PAK code for the feature license but that will just reference the upgrade.
If you are trying to find what the customer purchased that may not be possible.
You can get a system re-hosted by giving the existing MAC address and the new MAC, you don't need a PAK code for that.
You can try to look at the license files from the CLI if you have 7.1(3) or later.
try: file list license *
That should list the license files installed.
You can then download the files to a sftp server by: file get license
Once you have the license file names you can open a case with licensing and ask them if they have PAK codes.