To summarize what others have said:
Your upstream bandwidth is too low too support a G.711 call.
You will need to use a lower bandwidth codec in order to achieve two way audio.
A wireshark trace with net2phone would allow us to see which codec is being used.
As Tommy mentioned, as was going to be my next advise, you could set up a PBX account such that you use a lower bandwidth codec for your calls even if the end provider does not support said codec. This guide might be very helpful. »
forum.xda-developers.com ··· =2057887It is intended for and Android Cell phone over a 3G connection, but the principles are the same.