iPhone upgrade program - FedEx tracking number

I just realized Apple never received the phone I sent back through the upgrade program. I used the packaging and label Apple provided but lost the tracking number. This was sent back in December and I was struggling with some significant health issues at the time - I've never lost a tracking number before! Apple gave me a "return number," but has otherwise been unable to help. I thought the tracking number was pre-assigned and on the label when I received it from Apple.

I can't talk to anyone at FedEx without entering a tracking number into their system. I've spent hours trying to resolve this and am hoping someone here can help.