A2B doesn't get involved in the audio stream, so it can't be causing this problem. Perhaps A2B is influencing choice of codec or carrier to produce these results; if you were to test under identical conditions with Asterisk alone you would experience the same results. As ever, searching "echo" on voip-info
shows a lot of useful info, which should set you on the right track.
In the past I've also had this problem reported to me by one customer who was using a hands-free speaker phone with poor echo-cancellation. He then tried again using the speakers/microphone built into a laptop.