My wife's phone does this as well. It's because she deleted her phone number from the settings so "it wouldn't show" in case the phone was stolen, I guess. I'm pretty sure if you put your number into the right place it will show up in iTunes. On the phone, go into the "Settings" app, scroll down to "Phone" and select it. The first thing you'll see is "My Number". Select that and enter your number as "1 (xxx) xxx-xxxx".
Hope that does it for you!