If you are a current customer and replacing your current device with a new one your activation process will be slightly different.
During the activation process on the phone make sure you select "No, I'm replacing an existing device."
Enter the email address and name that is associated with your existing device. The next screen will bring you to a list of your current devices. Select the device that you are replacing. This will pull your number onto your new device!
If you have accidentally skipped this step and have two phones activated with two different phone numbers then you'll need to notify Customer Support by submitting a help ticket.