Poor quality when calling

Hi. I've a redmi note 3 pro 32g version. I've a big issue when i call or someone call me. The network bar is full, with 5 or 6 bars, but the sound quality is worst. It's like when you have a poor network. I've dificulties to hear and understand the person wich i'm talking with.

Any suggestions ???

Nb: when the network is on 4g it's ok. But when the network pass to 3g the probleme appear.

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