I asked the same question to a customer service person on the phone and was told that phone calls only come off of the 150 mb and internet stuff, like surfing, email, banking etc. come off of the free 5GB. He said its as simple as that, I hope so.
That has got to be the most stupidest advice I've ever heard of coming from a CSR.
Phone calls are phone calls. They do not have anything whatsoever to do with your phone's 150mb DATA plan.
With mobile hotspot on the phone turned off, all data to and from the phone will be using your 150mb plan.
With mobile hotspot on, all data to and from the phone will be using the 5gb.