Users Getting Logged Out of Mobile App; Not Receiving Calls.

Sometimes if users don't use the mobile app for awhile they get logged out and then the app doesn't ring. It makes the app completely unreliable because we never know for sure if it's even ringing when someone doesn't answer. For now I'm just forwarding all calls to the users' cell numbers because that always works, but then they don't get any of the features of the mobile app. Is there any way to ensure users stay logged into the app and receive calls at all times?