Hi Wendy
I have the same problem.
It seems to be some clever trick that Samsung have included to try and reduce ambient noise on incoming calls, and have basically made it so that on incoming calls the speaker is on very quietly, which makes a loop and causes an echo for the caller.
When you have answered the call, if you press the menu button, and press "noise reduction off" if will fix the echo problem for that call. unfortunately i haven't figured out a way to turn it off permanently yet!
Simon