Erratic Call Quality on iPhone 11 Pro

My iPhone 11 Pro is having frustrating erratic call quality that has been difficult to diagnose (and fix). Example: I can be home (not moving in a car) with good signal strength and call someone in their home (also with good signal strength) and the call can start out perfectly clear and fine. At some point you hear an electronic modem-type sound and from that moment the call starts to disintegrate. It sounds very electronic and like taling in a barrel. Then it starts dropping a syllable or two. And then a word. And then sentences. And eventually the call is so bad that neither party can hear the other. Once the degradation starts, it never self-corrects during the call. The only way to "fix" it is to hang up and either re-boot or toggle airplane mode on and off (presumably resetting network settings. Not every call is this way. But it is quite frequent (daily). It is definitely NOT poor signal strength, microphone problem or speaker problem. I have tried at least 3 different sim cards and had the phone company confirm their settings. But so far no fix. Has anyone out there run into this bizarre issue? And more importantly, does anyone have a fix? Thanks.