Reproduce for your Life

After getting to know the major players and the code in question, we
wanted to identify where exactly - in what stage of the connection
process - the problem occurs. 

To check if the software (pptp) is at fault, we tested the modem in
both in windows, and using a different PPTP client for linux (PPPoE, for example)

In order to check if the hardware is at fault, we tested pptp with a different modem.

Now, we know that the modem is working, since we managed to connect with it
in windows. We also know that pptp is working, since we managed to connect with it 
using a different modem. 

So what do we do next?