Hissing/buzzing sound when on "data" calls

[swy81]

My phone will make a either a buzzing/sound or a hissing sound from the earpiece when I'm using phone apps (Viber, Tango, etc.) to make phone calls.

When I'm on a data call, sometimes the earpiece will produce a constant buzzing sound. Or sometimes there will be no buzzing and it'll produce a hissing sound (after each word the other person says).

Using the regular phone, voice calls are clear with no issues.

I'm wondering if it's a hardware defect or network issue?

I'm on AT&T btw.