Can't Access iCalendar App while on a phone call.

When I am on the phone with someone, and I need to access my Apple calendar. I swipe up and go to the home screen where all of my apps are. I press the Calendar App and it just stays static. It will not activate. Any suggestions? The calendar works just fine when I am not on the phone, and other apps also work while I am on a call. Downloaded Google Calendar and it works well while on a call.