what phone do you have? Might be helpful if I decide I need to start telling my friends I no longer want to talk to them.
Most likely your phone doesn't have enough memory to run both this app and the phone app at the same time; possibly because of other apps you have running. I've certainly answered calls on my 5S mid-game without issues.
If this is a problem for you, kill other things you have before playing. There's not anything the app programmers can do -- overall system memory management on both Android and iOS is handled by the OS.