Just to clarify, are you referring to the phone app not opening on your device?
I would recommend testing your device in safe mode.
If your phone is misbehaving, it's possible that a third-party app is causing the issue. To figure out what app is acting funny, you can use the Safe Mode feature. In Safe Mode, your phone won't run any third-party apps. This allows you to easily identity and remove the app that may be causing the problem. Give this link a try:
Another thing to try is to for stop and clear the cache and data for the phone app?
Go to settings > apps > phone > tap on force stop > then tap on clear cache and data
Also, give this a try: Wipe the cache partition on your device. The system cache partition stores temporary system data. It's supposed to allow the system to access apps more quickly and efficiently, but sometimes things get cluttered and outdated, so a periodic cache clearing can help make the system run more smoothly. Give these steps a try:
Please keep me posted if you continue to experience these symptoms.