If you don't receive incoming calls when your phone is on Standby, then this is probably caused due to battery savings of the mobile phone.
For Android all apps are by default set to “Battery saving”. In order to receive incoming calls on the background, Android users need to adjust the settings in the phone.
Go to the settings of the mobile device
Go to “Apps” and scroll down to Qaller
Click on “Battery”
Enable the setting: “Allow background activities”
Please note that some phones requires a reboot in order to save all permissions properly.