FaceTime - How to direct communication to specific devices?
When you Facetime, the caller chooses either your phone number or the email address associated with your Apple ID. If it chooses the email address, it will ring both your computer, and your iPhone (as well as any other devices you have logged in with that Apple ID such as an iPad), and you may answer on either device. If the caller chooses your phone number, then just your phone will ring. In this case, the caller could use this to choose which device to ring you on.

As I'm sure you are already are aware, the device must be connected to wifi in order for it to ring (I believe there are cellular company outside the US that allow Facetime over the air as well), otherwise it will just ring and ring on the caller side without ever ringing you.