I had the same thing. Since I am an engineer and don’t really care about my warranty ;-), I decided to have a look. What I found in my case was that the antenna cable connector in the left lower part of the phone was not seated correctly on the small circuit board under the cover plate. I clicked it in place and since then no more dropped calls.
Might or not might be the same problem for people here, but it’s worth to have a look I guess.
Hope I helped someone here.
Regards,
Alfi