facetimeiphonemacbook pro
I tried to FaceTime my brother (who is using an iPhone 4s) from my MacBook Pro using the FaceTime app. The iPhone is on a 3G network, the MacBook is on my own Wi-Fi network.
The FaceTime app doesn't say that the iPhone is unavailable; it just keeps trying to connect, but the iPhone doesn't ring or do anything.
What might be the problem?
FaceTime requires a very strong network connection, so I believe your issue has less to do with the device, and more to do with the type of connections you trying to use.
FaceTime works best with a solid wifi connection on both ends, or at least LTE. If either device is trying to use 3G or 4G, you will struggle to establish or maintain a decent connection.
To test this, I would ensure you are both on a good wifi connection and try it again to see how it works. If not, then you have deeper compatibility issues.
P.S. I would recommend upgrading your iPhone 4 to iOS 7.1. It has improved performance over 7.0.x and will help alleviate further compatibility issues.
You should be able to do something like this with a product like IK Multimedia's iRig (or a generic equivalent... of which there are plenty).
The iRig splits the shared audio input and output on the iPhone/iPad headphone port. Plug a patch cable from your Mac's headphone port to the input on the iRig (you might need adapters to go from mini headphone to standard... 1/8" to 1/4").
Best Answer
I have the same problem. I assume that your brother's iPhone is on iOS 6 (which officially comes with the cellular FaceTime feature) and uses AT&T (which blocks this feature). I've heard someone say AT&T might start letting all iPhones use cellular FaceTime, but I believe the official word is that cellular FaceTime only works with AT&T if you have a Mobile Share plan or if you have an iPhone 5. Meanwhile you'll have to call him with Skype or Google Hangout, or he'll have to go to WiFi.