Using soft client from PPC - echo problem

I have installed * with FXO and FXS modules (TD400) and everthing is working fine regarding connection with other FIX or SIP clients. I’m using OpenVPN to connect from internet to the * …
But when I install soft client (X-Lite, ETalk, …) on my PDA (HTC TyTN II), and try make call with SIP or FIX client, the quality of the conection is very low and the echo is present. Echo is small in the begining of the call, but as time go it is incrising … I make adjustments with rxgain and txgain, use ztmonitor, and adjust the quality of the speach, but can’t eliminate echo.

Is there any general problem with connection from the PPC devices, or anything special that should be setup?

PPC also have OpenVPN installed on it, and as I notice the quality is also changed depending of the internet connection. With WiFi is ok, but when I’m connected thru GPRS or EDGE, quality is lower.