problem when a call begins

[leonialessandro]

Hi everyone, i have a problem with my lumia 820. When i make or receive a call, for the first seconds (say 10) the guy who is talking with me doesn't hear anything. It seems that if i use my phone for a bit before make the call this effect doesn't appear or it is decreased.
Thanks all.

 

[leonialessandro]

often when i "active" my phone it keeps few signal, after a bit the signal reaches the maximum power...