What does "No Service" or "Searching" mean on iPhones? I am new to iPhones and iOS. Since the update to iOS 12.1.4 my mobile connection breaks from time to time (seems to be a known issue) and "No Service" is shown at the top left of my display. I am not exactly sure what it means.

Previously, I used a BlackBerry Passport with BB OS10. There were two different information symbols for connection issues (with respect to mobile networks): a red cross indicating that no connection to any network is possible and another sign indicating that no connection to my provider's network is possible. If the second symbol is shown, I can make emergency calls (because there are other networks available). If the first is shown, I cannot do emergency calls.

Now, I am wondering what "No Service" means in iOS: no connection to any provider (no emergency calls) or no connection to my provider (emergency calls possible). My guess is that "no service" means the first and "searching" means the second. Is that correct?
No service means no calls are possible as you have no network,

Searching means no calls are possible and it is searching for your service set-up via your SIM card and Settings.