Network Lost

for the past couple of months, I keep getting this error

Network Lost

Your selected cellular network for "(null)" is no longer available. You can choose a different network in Settings.

2 options at the option: (Dismiss | Settings)

I am not sure what to do. I happens when I am at home, when I'm outside driving. When then happens there's usually a brief disconnection as well, I notice it when I am on a call when that happens.

Any idea what could be the problem?

Prior to that I have been with the same cellular network and did not have this issue for many months.