Have you tried resetting the network settings? I was having issues with dropped calls until I did it.
I am new to the iPhone. I noticed this phone does not sound as good as my previous Android phone.
I took it back to Verizon where we tested it with a"Apple Master" This is a descriptor he used since he had been through Advanced training He said my phone sounded normal. Anyway, he told me the Apple does not generally sound as good as the Samsung's HD calling feature. He told me Apples answer to Samsung HD calling is Apples WiFi calling, which he enabled on my phone.
I took the phone home and tried a call. The phone did not appear to connect to WiFi and the sound quality was unchanged. In the end, sound quality IMO is noticeably an issue for me I hope it's improved.