bad call quality

[kashir]

 I got a xperia z2 prototye. model 6503. 
Everything is working fine, except the call quality. i can hear and understand very well, but the other person has echoes, and very bad sound..
When I use a heatset, there is no problem. and it is almost good, when I put the phone on the table, with speaker on. The problem is worst, when I hold the phone on my ear..

Is it a hard- or software related problem?

[Uliwooly]

Do you have a protective case that might be blocking the second microphone ?

12 Second microphone 

[kashir]

no  second microphone is not coverd.  but i face same but more bad voice quilty problem in voice app like viber,skype 

[kashir]

Also to much bad voic qulity in viber ,skype,sip