Upgrading someone else line with App, Question

I am using an upgrade for another family members phone, but I won't be using that actual phone to preorder tonight. (Was gonna use the Apple App.)

Maybe I'm wrong but I thought I recalled the app pulling the number from the phone being used? Is that the case or not?

Im just trying to determine if I need to drive to go get the phone of the line I'll actually be upgrading or if I'm ok?