LINE messaging app incoming calls ringing sound problem

Hello.
I have problem with LINE messaging app incoming calls ringing sound.
Many times when phone screen is locked and I get incoming voice/video call, there is NO RINGING SOUND.
Only when I unlock phone screen and go in to Line app, I see there MISSED calls registered.
It not happens every time but is very annoying when I can not be sure that I hear all incoming calls.
Message notification sound works good with locked screen.
I did check all Line settings and Android settings, but did not find solution.

*LINE settings : Chats and Voice calls > Receive voice calls - YES
*LINE settings : Notifications > All options (Sound, Vibrate, Led) > YES
*Android settings > Sound & notification > App notifications under Notification > LINE > BLOCK OFF
*Android settings > "Do not disturb" option is not activated
*Android settings > Battery (Power) saving mode > OFF
*Android settings > WiFi settings > Keep WiFi on during sleep > ALWAYS
*Have Battery Doctor app installed, but all saving modes (Battery profiles, Battery saver, Schedule) are OFF
*Free space in phone and SD card > Both over 10 GB
*Have rebooted phone any times, no help

Please help me to resolve this problem.
Thank You

I have Samsung Galaxy Note 3
Android 5.1.1
Line version 6.4.1

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