Q: How do I change my iMessage so that it shows my phone number rather than my email address?

What error are you seeing? Activation of iMessage at times can take up to 24 hours. You need to ensure that you have cellular data on, and that you have texting plan. Make sure that US Cellular is a supported carrier. Activation requires the sending of a hidden international SMS to the Apple Servers in the UK, and receiving response. You will not be able to use iMessage until it becomes active.

They are seeing the email because of the incomplete activation. I suggest you turn off iMessage and FaceTime. Do a reset on the device, holding the sleep/wake and home buttons together until you see the Apple logo and then release. Once the device restarts, go back to iMessage and turn it on and see if it activates. Do not try anything else or sending a message until you see it showing activated. Then you can try going into Send & Receive, and see if the phone number is listed there and it is checked.

If you continue to have problems with activation of iMessage and have left it on to activate for 24 hours and it has not completed activation, then contact Apple Support. If you get an error when trying to activate iMessage or FaceTime - Apple Support

if you can send texts with your email showing, imessage is working.

go to settings/messages/send+receive. check your mobile number at 'start messages with' at the bottom. voilà.