My husband and I are having the same issue that just recently developed with each of our phones. This JUST started happening, and it doesn't seem to matter if we are home or out. He has a brand new iPhone 11, and it JUST started happening with my iPhone 6s+ a few weeks ago, too, that I've had for years working reliably. He has been on the phone with both Apple and Verizon repeatedly, and has been sent new a new phone (obviously not the problem, since my phone has been problematic all of a sudden), and been told that we need a WiFi booster (again, not the problem, since I have full bars and he alternately has full/none AND, I've been operating on data, AND we have Comcast for Internet). What gives??? People can get through to us, and then when we answer or when we call out, nobody can hear us on the phone, and then sometimes they can. IIt happens on speaker and not on speaker. I don't use ear buds - hate them - they fall out of my ears, and I have to go chasing them all across the floor. I like having my earphones tethered to my phone like a runaway strap (one less thing to charge + I can use them on planes, too - if I can ever travel, again - it's why I won't upgrade my 6+) In any case, this is very frustrating. If anyone can provide insight - I suspect it has something to do with signal wave length, like back in the day when cordless phones were having interference because they were on the same "channel" as other devices, but I'm having trouble getting anyone to listen to why it cannot be a "phone" issue or signal strength issue.
You certainly have been put through the wringer, with all the testing tried. Ahead of us our questions that may have been asked before. You mention he was sent a phone.
Has this been activated? If so, then does this happen on the replacement? On the iPhone 6s+ , has the audio jack been looked into for any possible debris?