Why no simultaneous voice and 4G LTE data on S7?

[AC Question]

I bought the new S7 at Costco on May 7. Worked great on test calls and data. Drove the hour home, phone won't allow simultaneous voice and 4G LTE data. In the valley at the store, the Advanced Calling is set to HD Voice & Video. In my area (still Verizon area), it automatically reverts to HD Voice only, and goes to 1x when on a call. I spent an hour in the store last Saturday, and another hour on the phone with Tech Support yesterday. No answers available. Was told my phone was defective and to return it to Costco. Got to Costco (an hour away) and phone worked flawless. My S5 worked without any issues at all. The new S7 doesn't let me use voice/data simultaneously and it's affecting my work. Any thoughts?