If you installed iOS 10 you'll be surprised at how different it is. Since it's a major update some basic things are affected, including phone calling. Most every non-techie I know doesn't backup settings to their computer via iTunes or to iCloud. After an iOS 10 upgrade, your settings are likely lost. Sorry, this means entering some settings manually.
Additionally, you're likely to experience frequent call drops. Because settings have changed, you need to reset your Network Settings to fix some call drop problems. If you use WiFi calling, you may not be turned on anymore. Even worse, if you have an AT&T microcell it will probably not work either and really screw up your call stability.
All of this can be fixed using the Apple procedure documented here: Make a call with Wi-Fi Calling
If you can't turn on or use Wi-Fi Calling, make sure that your carrier offers Wi-Fi Calling and that your devices have the latest software. Then, follow these steps if you can't turn on Wi-Fi Calling or make a Wi-Fi call. Wait two minutes before you go from one step to the next.
- Go to Settings > Phone > Wi-Fi Calling and make sure that Wi-Fi Calling is on.
- Restart your iPhone.
- Connect to a different Wi-Fi network. Not all Wi-Fi networks work with Wi-Fi Calling.
- Turn Wi-Fi Calling off and then on again.
- Go to Settings > General > Reset and tap Reset Network Settings.
The goal is to get your phone to look like this: