Default Help please. Problem with signal, phone.apk

Hello.

Just bought Galaxy Ace II GT-I8160, on 4.1.2 stock, I8160XXNB1.
I was thinking of rooting it, but a problem occured and now I reconsider because of the 2 years warranty.

The problem is with calling someone. I call, he responds, we talk a various amount of time and then suddenly I can't hear the person, he can't hear me, the call seconds still move on, but after 5-10 seconds, the phone blocks. The "back" button doesn't work, home button doesn't work only after a few seconds. I can only lock/unlock screen, but the whole phone is a mess. And surprise, in the end, I get a "com.android.phone.apk stopped" and also the "no network" sign in the notification bar. After 10 minutes, the problem dissapears. This doesn't happen every day. I'm guessing that my house is to blame, because i have some stupid rooms where I get only 1-3 bars of signal, but why does phone.apk stops?

Still, is there a software or a hardware problem? How can I solve it? I hope is something software cause I need the phone in the next 2 weeks, can't take it to warranty.

Thanks