We have tested this on both GSM and CDMA iPhones, both having the same behavior. Since, the traditional phone is a core feature of the device, it will always have priority. So, if you are on a traditional device phone call and you receive an incoming StarPhone call, if you are looking at the screen you will see the alert (notification) but no audio since the iPhone traditional phone is consuming the audio and will not share it. Additionally, you can switch over to the application and see the accept or decline screen. If you accept the call, the caller will not be able to hear you and you will not be able to hear them. Our recommendation is for you to decline the call and let it go to voicemail.