I have a Mac Mini using OSX Mountain Lion, an iPad 2, and an iPhone 4. I tried to receive a FaceTime call on my Mac but it was telling the caller that it was busy. I was however able to call her iPhone from FaceTime on my Mac and we connected that way. After our video conversation I tried testing this by calling my Mac from my iPhone. I also received the message that it was busy. I should also add that when I first received the call I had my iPad FaceTime turned on. The iPad was ringing but not the Mac. So I turned off FaceTime on my iPad and had her call me back again using my email address. As noted it said it was busy. Please help!
Do you have different FT addresses on your Mini, iPhone and iPad?
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
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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.