How to differentiate between a full signal icon and an empty signal icon on my Andriod Device?
The "no network" in the notification tray doesn't mean there's no signal: it means your phone isn't allowed to use the network it's connected to. For instance, this might happen if there's no SIM card. In this case, your phone's not allowed to make normal phone calls, because it can't identify you as a customer, but it still connects to the network and can make emergency calls.

It might also happen in an area where your operator has no coverage, but there is another phone network there. Again, you can't use it because you're not their customer, but your phone can connect to the network and make emergency calls.