Sign out of messages on ipad, reboot ipad by pressing and holding both the home and power buttons at the same time until the apple logo appears on the screen, then let go.
Go to appleid.apple.com on iphone, manage account, verfiy phone number is same
Now go back and sign into messages on ipad, using same apple id you just verified on iphone
Messages/facetime and assigning phone numbers
iMessage will not accept my mobile number so cannot get SMS on ipad
Even if you get iMessage set up properly using Grumpy Gator's instructions, you still will not get SMSs on your iPad, only iMessages (from other iPad, iPhones, iPod Touchs or Mac computers). If you need to get regular SMS, you'll need an app from the App Store such as TextFree.
It looks like after fixing their security issue with Apple ID's, it no longer works at all for some accounts. This prevents iMessage and Facetime from linking to a phone number. What happened to Apple? I used to be fine paying a premium price for a premium product, but now, everytime they fix or upgrade something, it's done in a Microsoft kind of way. They're very close to losing another long time customer.
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
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
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
Unable to use FaceTime and iMessage with my apple ID
My Facetime Doesn't Ring
For 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