Callers say my voice turns to static while on a call. LTE related? iPhone 6-AT&T

Question

Callers often tell me my voice turns to unrecognizable static while on a call.  This is an iPhone 6 on AT&T.  This usually happens anywhere from a few seconds to a minute into a call, and is very frequent, upwards of 40-50% of calls.  Happened on the last 3 calls I just got today, hanging up and calling back clears the problem.  This is not acceptable when you run a business and get 30-50 calls per day.
When this happens I continue to hear the other party fine, however they typically say, "I can tell you're talking but I can't understand anything you're saying".  Signal strength is fine, I'm 800 feet to the nearest tower.  Went to Apple store, guy said there was an unusual amount of S/W errors on the phone, they had me do a complete software reinstall.  This did not fix it.
Called AT&T, they analyzed the nearest tower for errors and found a possible issue with LTE at that location, but they were not sure.  Coincidentally the guy at the Apple store told me to try turning off LTE.  This might be a cure because it (so far) hasn't happened while LTE is off.
Anyone else had this?  Suggestions?

Answer 1

I did try setting LTE to Enabled and Data Only but I still had the problem.  Seems so far that LTE needs to be completely off or else the phone's outgoing audio often becomes badly corrupted. I'm still trying different settings but it's hard to do when you run a business.