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. When the traditional mobile phone receives an incoming call the device will start to ring and audio on the StarPhone will be suspended. If you answer the traditional phone call, the StarPhone caller will hear dead air (see alternative below) until you hangup with the traditional phone, at which time audio will resume on the StarPhone. Likewise, if you decline the traditional call, audio will resume on the StarPhone. Our recommendation is for you to decline the call on the traditional mobile phone immediately which will immediately restore audio to the StarPhone call.
An alternative is to provide the StarPhone caller with Music on Hold when your call is interrupted. Access "Settings" and select the ("Hold on Interruption") option that will place the StarPhone call on hold when receiving a traditional cell phone call.