Here is an example.
My friend is on T-Mobile USA. Hes does not have coverage issues, however inside his home (probably due to the construction stone) the GSM signal is very-very weak. His office is located at the basement of that same house. There he has no signal at all meanwhile he spends a lot of hours in this office.

He is now using a UMA ebable phone. So now when he is on home his is getting T-Mobile coverage over his own WiFi network. This means he is getting full network signal over wifi even in his basement that the actual network has no bars at all. All network calls and SMS are routing from Wifi using UMA.