I have my WiFi-only iPad Air set up to make and receive incoming phone calls to/from my iPhone via audio FaceTime . For the past couple of months, if the iPad is charging and a call comes into my phone, both the iPhone and iPad ring (as expected). If I answer on the iPad everything is fine, if I answer on the iPhone, a repeating tone plays from the iPad and the only way I found to stop it is by unplugging th charging cable.