VOIP Buster has voice delays

My friends all been using VOIP Buster, with no problem and they recommended it to me.

Although they have problem-free calls, I never had one, Everytime I call any destination [whether in Europe or U.S] , I hear the recepient's voice but they don't hear me at all or hear me with delay of approximately 10 seconds.

I tested my sound system by Skype and Gizmo, it worked well and I made some successfull calls with them.

But my problem is VOIP Buster itself doesn't work properly, and I keep blaming myself for paying 10 euros just to test it

Please help me fix this problem, tell me all the possibilities and I'll check them one by one, I'm even ready to buy a new ADSL Modem to fix this.