Monitor Network Availability (for phone calls)

Obviously my network carrier has some problems. Though my mobile phone shows two or three bars out of four for my mobile connection, I don't receive any phone calls (not available). Is there some kind of app or service that notifies me, if I cannot be reached by phone? WiFi is available, so it wouldn't be sufficient to just ping an internet server. Any other ideas? I am using a Samsung Galaxy S5 mini (SM-G800F), Android 6.0.1, Voice over LTE is not supported on that device.