gtalk not supproting voice chat....

I confronted this problem when I was using gtalk on a dialup connection @40Kbps I couldn't transmit any voice signal on a call though I was able to hear what the other person was saying..... I am sure there is no problem with my mic as I was able to use it in yahoo Vchat....

Help me here somebody in what is the prob. n if there is a soln.??
I am totally bamboozeled... thanx to gtalk..'..