the phone works for me, unfortunately not always

I installed by a professional friend to me the JVC KD-R851BTE. Everything laid and connected clean. Radio functions folded right away. Pairing with my I-Phone 4S takes a little longer (2-3 attempts). The test call of colleagues from his smartphone arrived and I was able to take the call on the acceptance button on the unit. Stream music from I-Phone also works flawlessly. If the navigation (Navigon) running the music for announcements is interrupted. So far so good.

In everyday use, I put unfortunately determined that the device does not always work. There comes a call in, music is low and the signal can be heard through the speakers. I press the answer button and nothing happens, the call is then after several failed attempts, unfortunately, gone. Unfortunately, that's now happened already often. So I test again on my doorstep, call myself to the Home phone in the car and it works again. I caught a Monday device? Even after repeated reading the manual I can find no solution.

After Apple i-Phone update everything works fine