Call sound problem 950XL

[GusarP]

Hello.

I friend of my gave me a new from the box 950XL for testing. I was still original packed. I upgraded it to the last
possible update.
Email sync, Secure WI-FI access is working OK, photos, VPN, sound from music OK, Company profile etc.
The phone feels good and I thought it was all OK.

But after a couple of days in on-line mode (i had to change my SIM to nano) I noticed something was wrong.

When a made a call, and not every time, the sound goes from good to very poor (hissing,etc).
And then there is like a very high pitch sound (star trek / star wars) phaser gun sound rly: and goes through my ears (very damming sound).
After this the sound is gone I can not hear the caller, but the caller can hear me. But after i can switch to loudspeaker and then I can hear him.
After that every call doesn't have the sound and I have to use the laudspeaker. But when a turn of and on its working again for a couple of calls

I did factory reset the phone 2 times. It was OK for a couple of days, but its back again.

What do you all think is it some SW problem or HW problem. Did somebody have similar problems.
Should I take it to warranty service check-up. Thank you for your answers.

See you all.