I believe they were trying to get you to mention the idea of "black box" testing. It is test where a tester performs testing without looking into the code. OR A testing method where the application under test is viewed as a black box and the internal behavior of the program is completely ignored. Testing occurs based upon the external specifications. Also known as behavioral testing, since only the external behavior of the program is evaluated and analyzed. So you would explain that you would only be testing the external behaviors of the cell phone.