Random incoming calls with no way to answer, not showing caller ID or in recent call history... how do I fix this?

I thought my Mom was crazy when she called and said she couldn't answer her phone until she came to visit and it happened several times in front of me. Her phone would ring and there was no display of who was calling, no way to answer the call and it didn't show in the call history. I would test call her and my caller ID would show and I would show up in her recent calls. I did the proper troubleshooting and made sure her Do Not Disturb was off and it was. Under Do Not Disturb I went to Allow Calls From and Favorites was checked so I thought this might be the issue so I selected the Everyone option. We even turned the Airplane Mode on and off to see if this would help. She then received two calls back to back and the same thing happened again. The phone would ring nothing would show on the screen as an incoming call and nothing in recent calls. I would call her right after and no problem. I turned on the Full Screen option under Incoming calls for her and I even turned on Announce Caller but she received like two more calls and nothing showed up. I told her we were going to have to try to reset her phone and she told me she already took it to the AT&T store and the gentleman there reset it for her. What else can I do to fix this for her? Could she have downloaded an App that could be calling her phone?