Voip dialer getting cross connection..?

-I have broad band internet connection. (Aztech router). We are three people using this connection. also using voip dialer. The problem is, when i am using voip dialer,if there is someone trying to call through voip dialer same time from other PC in my network, my sound can hear that guy and my call getting co lapse. also vise verse. please give me a solution . thanks in advance.