A couple of months ago people suddenly weren't able to hear me when speaking on my iPhone 5s inside my house and on my outside porch (close to the house). I finally went to a Verizon store and they suggested it was a "damaged antenna" so I got a new iPhone 6s. I still have the same issues! What's even more frustrating is that my husband has an iPhone 6s Plus and has no issues with service in our house. Help!
Did they give you a brand new SIM card when you got the 6S? If they used the same one from your 5S (which they should not have) then you have a bad sim card.
Do you put on any cases or anything. Certain ones may block your microphone just right to make it sound like its cutting out.
Otherwise its a long shot but you may have gotten a defective unit and could have it replaced via apple warranty. The 6S Plus does have more amplifiers (because of its size) but if it was sudden then its most likely your sim card they do wear out over time.
I hope this helps
You may be having an issue with LTE voice calling. Check to see if the phone is set to "Voice & Data" in the settings menu. If so, set it to "Data Only" & see if that helps.