I was on a support call with Apple, checking to see if they received my old phone that I had shipped to them. The iphone I was using to make the call to Apple was plugged into charge while I was on the phone call.
The phone was also in low power mode. When the charge level reaches 80%, low power mode is automatically disabled, as seen by a notification popup.
When the notification for Low Power Mode being disabled was displayed, the phone call got dropped. This could be coincidental, but I filed a bug report anyway. Let them try to duplicate the issue.
Unfortunately, I had to call back and sit on hold again. I asked the service person to make a note in my file that the call was dropped, while the original service person had me on hold.
I wonder how often the calls get dropped when users call in on their iphones 
BTW, I had a full strength cell signal when I made the call.