Do you have an iPhone? You can only use a phone number from an iPhone. It will not work with any other cell phone number.
Just so that you know .... Messages only works with other iOS devices or Macs running Mountain Lion. The other user that you want to reach must already be setup and registered to use Messages on their device as well. If they haven't registered Messages on their device, you will not be able to reach them.
You should still be able to link your phone number to Messages and FaceTime however. See if this is of some help to you.
I think basically, you have to sign out, turn the apps off, and then start all over again in order to link your phone number, but you can read the article for yourself.
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