Here’s what I recommend as it just finished activating.

Connect the iPhone to the another iTunes, preferably the iTunes for the lucky one who got the iPhone. So it could either be another computer or another account on the same computer.

Then start the activation process and select add new phone to an existing plan. Enter the phone number of the phone with an existing plan and then complete the registration process

I added one iPhone last night around 11pm. At 8:50am this morning I received an email notifying me that the iPhone was activated and showed me my new number. I then connected to the iTunes account and the iPhone started and completed the activation process.

The I set the Sync preferences and after a few minutes, all the information had transferred and the iPhone was ready to go.