Sign out of iMessage on both devices. Sign back into iMessage on your iPhone. Wait until it completely activates. Sign back in on your iPad using the same iTunes account. Your phone number should now be linked to iMessage on your iPad.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
iOS: FaceTime is 'Unable to verify email because it is in use'
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
iOS 6 and OS X Mountain Lion: Link your phone number and Apple ID for use with FaceTime and iMessage
How to Set Up & Use iMessage on iPhone, iPad, & iPod touch with iOS
Troubleshooting iMessage Issues: Some Useful Tips You Should Try
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
FaceTime, Game Center, Messages: Troubleshooting sign in issues
Unable to use FaceTime and iMessage with my apple ID
How to Block Someone on FaceTime
My Facetime Doesn't Ring
Send an iMessage as a Text Message Instead with a Quick Tap & Hold
To send messages to non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8
How to Send SMS from iPad
You can check the status of the FaceTime/iMessage servers at this link.
I am having the same problem. I am not able use my phone number in iMessage on my iPad. It works on my iPhone but only email works on my iPad. I signed off of iMessage on both devices, signed back on through my phone, then through my iPad. Still no luck. The only thing I can think of is that I'm using 2 different apple ID's. One for my phone and one for the iPad which is primarily for work. Is this the problem? If so is there a way to use both ID's together?
Yes, the two different Apple IDs are the problem. You can only use the iPhone number on the iPad if both devices are signed in using the same Apple ID. No, there is no way to tie the two IDs together. However, you only have to sign in with the same Apple ID in iMessage. You can continue to use a different Apple ID for iTunes & App Store, and iCloud.
How does my wife add her iPad's phone number to her iMessage?
To say the iPad is not a phone and therefore you don't need to is wrong on so many counts. She has a 3G SIM and they send text messages to it to let her know when her balance is running low, when she has topped up etc. she also needs to be able to send messages from this number to get them to add the data bundle.
Why is this so difficult? She does not have another device with iMessage. How can she add her number so she can manage her account?