First, reboot the phone, and run the app manually. Make sure you can make calls, then test incoming calls.
If the phone fails to ring for incoming calls, note whether it shows the call if the phone is unlocked and the 3CX app is open. If it just stops ringing when the phone is locked, please let us know.
Removing the app, rebooting the phone, and then reinstalling may solve issues. We can work through this with you if necessary.
If these steps do not resolve the issue, here is the info we will need:
Brand and model of device (phone, tablet).
Version of the operating system (we recommend keeping it up to date.
Is the 3CX app up to date? Removing and reinstalling would assure that.
Is the user having this issue when the mobile device is on wifi, on cellular, or both?