Why This Happens to Incoming Calls Only?

i use huawei TAG L21. on locked the screen if someone calls me then my phone does not rings and no incoming call shown on phone. after missing the call screen light blips . i missed too much calls. While if the screen is on then incoming call rings and shown on screen properly.Remember one thing.........this doesn't happen for all other notifications, like SMS , Whatsapp messages and calls etc.
pp

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