Background noise during Calls (Iphone 8 Plus)

I just bought an Iphone 8 Plus. (last update 11.2.6 installed).

When I'm in a call I hear a background noise when the interlocutor is speaking, it's like the iPhone amplify the sound and appears a ("sea waves" = "ssshhhhhhh") sound when there's sound by the other side. When the interlocutor is complitely in silence, there's no noise, but as soon as speak, the noise reapears. It's not a hardware issue because the sound only appears when the other side talks. It's a little anoying..

The other side hear me crystal clear with zero background noise, tested with 5 phones...

I've restored from iTunes and the problem persist.

Anyone else with this problem? Any solution?

Thank you.