Why do other peoples voices echo when they call me?

I got this Nokia Asha 503 and I really like it. However, other people complain that when they talk to me on the phone, they hear multiple echoes of their own voice. On my end it sounds just fine.

I called my cell phone from another phone and when I talked it did really echo. Only the other person`s voice echos, mine doesnt.

Is this something wrong with the phone or with the network? I am with AT&T.