You can try is to switch off Do Not Disturb option when you find your iPhone won’t ring. Sometimes you accidentally switch on Do Not Disturb option, which silences notifications, alerts, and calls. Your iPhone will not ring, but it still allows the phone calls to come in and as you won’t hear the sound, you will see missed calls. In some cases, the user accidentally turns this feature on without even realizing. You can check at a glance to the top-left side of your iPhone screen. If you see the moon icon, it means the Do Not Disturb mode is activated. To disable it, follow the steps below:
1. Swipe up from the bottom of the device to bring up Command Center.
2. Tap the Moon icon to turn it off.
Here are also some other methods to help you solve this
iPhone won't ring problem.