C# application using GSM+GPRS

Hi,

I'm using a C# embedded application which is doing the following tasks:
-GSM always connected
-GPRS if needed

So, my PDA has a modem in order to do those actions.

The problem: sometimes, I obtain one of the following errors listed at the
page:
http://www.modemsite.com/56k/dunserror.asp

I don't know how I can reproduce those errors. Does anyone has an idea ?

Thanks in advance.