Lets try to start over.
Create a brand new application with the PPP example code avaiable with the PPP library documentation.
First try it with the siemens modem that worked before then try the other modem.
And show me the code you are evaluation with.
[quote]Even “funnier”, plugging that board to the EMX devboard makes the EMX crash (rebooting and restarting all the time) as it seems it cannot provide enough voltage.
However plugging it to the FEZ Cobra, the power supply seems to be sufficient. (we checked several cable combos)
[/quote]
EMX Dev Sys has a display which withdrow considerable power. You should try it with 6V external power or connect the USB cable to a powered USB HUB.