LTE setting can negatively impacts Conference calls

Why are my LTE setting impacting conference calls:

Two weeks ago, I purchased a through Apple a 6S 64 iPhone and my carrier is AT&T; and now I've noticed that I no longer can conference my calls the same way. Technically I can conference them but it's in a downgraded manner. Under Settings, under Cellular, I went to Enable LTE. Already Voice & Data was preselected for me, but if I downgrade to Data Only or Turn off LTE, then I get the full conference call functionality back. I've contacted both Apple and AT&T for help with this problem. Apple help me by suggesting I change my LTE setting, and it worked. They warned me though that the Data Only feature or No LTE would mean a decrease in call quality, and suggested that I speak to AT&T. AT&T will not address it at their Tier 3 yet; they think it's an Apple software upgrade glitch. Does anyone understand this yet?