For some time, Verizon did not support simultaneous personal hot spot and cellular talking on iPhones. This was a hardware limitation on the iPhone at first. Then, the release of the iPhone 6 allowed voice over LTE (VoLTE) technology. VoLTE allowed any iPhone 6 or later to use the hot spot and cellular talking at the same time.
Only iPhones 6 or later allow for simultaneous Hot Spot and Talking the same time on Verizon.
Additionally, you must enable Advanced Calling 1.0 and HD Calling in your Verizon account in order to take advantage.
Finally, ensure you have enabled Voice & Data in cellular settings. Go to Settings -> Cellular -> Cellular Data Options -> Enable LTE -> Voice & Data