USe case ::
OUT OF COVERAGE: Where operator does not deplyoed their networks. ex: when you travelling in train. in middle of journey you wont get signal, that time when some one tries to reach you then they receive " the customer is out of coverage area" voice note from the operator. Here two persons of same network experience same out of coverage.
OUT OF SERVICE: This is operator stopped services for you. Just like over due of your post paid bill. Means even operator deployed his network but forcebly stopped providing services to you. Here two persons of same network does not experience same. The person who blocked by operator get out of service and the person who clearing his bills on time get connectivity.