My iPhone 8 is on Sprint. It’s running the latest iOS12.2 However a few days ago Sprint pushed a carrier settings update to my phone. Since then I was basically unable to use the phone app to make calls. Whatever window it was on last (Favorites, Contacts, Recents, Keypad or voicemail) it would be stuck there. Attempting to switch by pressing one of the buttons would either crash the app out, or take up to 15 seconds or longer to switch. I went to Sprint to ask and all they could suggest are all the things I had already done, hard reset, reboot, make sure I didn’t leave any apps open etc.
Their only suggestion was to make a full backup, then restore to factory settings and then reinstall apps one by one and see if I could isolate the problem.
Got home and in preparation for that started verifying that I really needed all apps on my phone. None could be deleted so I started looking at the voicemails and recents in the phone app. I had a lot of stored voicemails some going back to last August. I’ve been deleting them after verifying that I had captured all the important info from them. I’m down to about 15 left and the app seems more responsive. It’s still about 30 seconds to come up initially but it hasn’t crashed out on me in a while and I am able to switch between recents, voicemail and keypad. I’ve done a full backup and have again hard reset the phone.
Just passing this on in case it helps. Will update if things go sideways again.