Do you have Do Not Disturb on by chance? Airplane mode would prevent you from making or receiving calls, blocked contacts should prevent calls from those contacts and we'll just assume that isn't the case, leaving DND as the only reason I can think of that your device wouldn't be accepting incoming calls.
Well almost, did you recently change carriers? Is the device compatible with your carrier?
Is the device factory unlocked? Meaning did you buy it as an unlocked device or did it come carrier locked/branded?
Have you ever been able to make and receive calls on this device or is the device new to you? If it is new, how did you buy it? From a store, private sale, factory direct?