Dingtone needs to stay in the background to receive incoming calls and messages. Please do not terminate the Dingtone app after you close it. Some third-party battery-saving/memory-saving apps may terminate Dingtone. Please add Dingtone to their whitelist.
Please also make sure notification is enabled for Dingtone. Enter your Android phone's Settings > Application Manager, find Dingtone, enter App Info screen for Dingtone, and make sure "Show notifications" option is ON.