How to set our app as default Phone app in GXV3380



I have a phone app for the device GXV 3380. The app is working fine with the device. But I couldnot able to set my app as DEFAULT phone app in the device. There is no options in
APP -> Default APP.

This plays a role in handling the offHook and onHook events.

Is it a bug in the device ? Because the same app was set to default with GXV3370.

Could you please look into the problem ?