Use an email address (gmail?) 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
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
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
For non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8
Thanks Tom, but I read and tried everything in those documents already. My problem is a bit different that what others report. I can get the phone number registered. I can send from the number. I am just not receiving iMessages to the iPad unless I've sent an iMessage to the contact. Once I send an iMessage to the contact I become part of the 'conversation' but unless I send an iMessage from the iPad I don't get any of the communications.