Hello gabbyurmetz,
Thanks for posting your question. It sounds like you have a trade-in question. I'll be happy to see how I can provide some helpful information.
To clarify, are you referring to the part of the trade-in process where it's asking you to enter your contact information? If so, you can enter any phone number where you can be reached.
On the other hand, if you're asking about a serial number instead, you can locate it using the suggestions in this link: Find the serial number or IMEI on your iPhone, iPad, or iPod touch
Have a good one.