[Q] N6 - No incoming & outgoing Calls, mobile data gone. topic

Having my Nexus 6 for almost a month. Things working perfectly in the starting(or after reset), but after two or three days, No incoming calls, no outgoing calls. No mobile data. Cant realy tell it as a mobile sometimes. Wifi is working good always. After reset, it is working properly for two or three days. Whenever it is not working, I am able to see a !-mark near the tower bar.

My mobile always connected to 4G when no wifi.
No Custom ROMS and no root. Plain Android given by google.

Anyone having this issue. Any suggestion?