Who I have the same questioned this topic

After over a year of having this happen, I've had my phone just start to screech/squeal a high-pitched noise whilst talking.  The person on the other end doesn't hear anything amiss.  It's loud enough to physically hurt my ear.

iPhone 6s+, using wifi and VZW Wifi, iOS 11.4.  Since this latest software update, I now also randomly get "No Service" in the upper left that will magically re-appear after a bit.  I have not had the screeching when off wifi.

Wifi consists of Google wifi mesh system with 2 nodes.

I saw one person solved their problem 3 years ago with iOS 9 but the menu tree has changed since then so that I can't find my way in to turn "advanced voice" off (perhaps this option is no longer available).

I'm about to get rid of this phone unless the issue can be resolved.