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
My mistake, if you have a phone number associated with your Apple ID, your iPad can receive iMessages via the phone number but only if you've set iMessage up with that particular Apple ID.
Wait! What? Any old phone number can be linked to iMessage? I don't think so, I only think iPhone numbers can. There's no way an SMS text on your ancient Motorola could magically end up on iMessage. At least, as far as I understand the magical world of cellular networks.