At times, iPhone calls may divert to voice mail before you get a chance to answer them, or you may not know that you've had calls until you see that you have voice messages. This may be due to your iPhone's settings or, in some cases, to issues with your cellular network connection.
Ring/Silent Switch is Off
Calls may divert to voice mail if you don't hear your iPhone because its ringer is switched off. Even if you didn't turn it off on purpose, this can happen by accident if the button gets pressed in your pocket or bag. Look at the "Ring/Silent" switch on the top left side of your iPhone. If you can see an orange line, your ringer is off and you should flip the button to turn it on.
Call Divert Settings Are On
Your iPhone has a couple of settings -- Airplane Mode and Do Not Disturb -- that send incoming calls direct to voice mail. You can tell if these are enabled by looking at the status bar at the top of your iPhone's screen. If you see an airplane icon, Airplane Mode is on; if you see a half-moon icon, Do Not Disturb is on. To switch them off, swipe up from the bottom of your screen to open your Control Center. If either setting is on, its icon is highlighted. Tap the icon to turn it off and swipe the panel down again to return to your Home screen.
Carrier Settings Need Updating
Sometimes, carrier settings can cause connection problems between your iPhone and your carrier network, so it's worth checking if these need updating. Tap "Settings," "General" and then "About." If there is an available update that you haven't yet authorized, you'll see a prompt message. Tap the "Update" button to start the process. If your settings don't need updating, try refreshing your network connection by turning your iPhone off and on again.