Why do calls disconnect on my Nexus 6P when put on LTE?

[AC Question]

Hi, I own a nexus 6P. In Andriod 6.0.1 I encountered a problem with calling when I put the "preferred network type" as LTE. After I upgraded to Andriod 7, the issue is still there. Every time I make a call or receive one, it does not ;last more than a few seconds. However, when I set the "preferred network type" as 3G, there is no issue with calling but with the internet. I get 2G on 3G network type. Please help