This is usually an issue with your carrier. If the carrier signal doesn't trigger the phone that a call is incoming, the phone won't ring. Check with your carrier to see if they have your account configured properly.
This may seem like silly suggestions but i would double check to try to get to the bottom of this issue.
1. Is airplane mode on ?
2. Is Do Not Disturb enabled ?
3. Have you tried calling from different numbers or just one ? (if just one ensure you havent accidentally blocked this caller)
4. Do you have anyone in your household which uses the same carrier as you do ? If so, i would test out another active SIM card of the same carrier in your device. Is it going straight to voicemail as well? If it rings i would follow up with your carrier. They may be a service interruption on your account.
I hope it was a quick fix and your service has been restored
Sorry for the issues
You may also want to check if call forwarding is turned on.
*73 to turn off or in the settings of the phone app in other phone other phone settings.
The first step I would take is to go to settings>general management>reset>reset setting. This will reset all setting on your device. Please make sure you do not hit factory data reset this can erase all info from your device. You will have to redo a lot of settings you put in place but can quickly pin point if there is a settings issue with out searching through for many specific things. If this doesn't work I would suggest going over to your cell phone carrier to see if there is a features issue on your account or simply getting a new sim.