How can I get iMessage to verify my phone number?

iMessage won't verify my phone number. When I try to select my number for messages to be sent, it's greyed out. So I receive my iMessages over email which I don't want to do.

I've tried: turning iMessage on and off, resetting network settings, rebooting my phone and nothing's worked.

Any ideas?

Looks like you tried all the basic troubleshooting steps. Heres the next step:

1. Make sure your data is backed up, either iTunes or iCloud.
2. Turn iMessage off (make sure to do this).
3. Erase all contents and settings (this will wipe your device.) Settings > General > Reset > Erase All Contents and Settings.
4. Set the phone up as New (make sure to do this)
5. Turn on iMessage, wait for it to activate. Once it's activated erase the phone again and restore from your backup.