I have a similar issue on my Macbook Pro, running Mountain Lion. Facetime works perfectly on my iPhone 4, and also works great between my iPad 2 and my phone. The problem is when I make a call to my Macbook from my iPad, there is no "accept" button, only an "end" button, and two grayed out icons for muting and enlarging.
The only option it gives me is to end the call. There is no problem going out from my Macbook to my iPad, or to my phone, or from my phone to my Macbook. The problem only occurs when I call from my iPad to my Macbook. You may ask why I would want to do this, but I want to have a larger screen than on my phone, when I am away from my family, and would like them to call from the iPad.