Hi,
I could not get my standard fax machine to send or receive using VoIP either, but the modem works fine. You will find this hard to believe, I did when I read it on a Linksys forum, but here in the US we have two types of RJ11 modem/phone leads one has 4 wires connected and the other has 2 wires connected, faxing over VoIP lines will not work with the 4 wire one

and this was my problem at the first attempt

I have tested both types and it doesn't work with the 4 wire one (or at least not for me). I am assuming that you are located in Germany so I don't know how your RJ11 cables are wired. I believe my problem with the standard fax machine was that the SP2000 (or PAP2) times out before the fax finished dialing, I have the same problem with a phone connected to the other port if I don't start dialing quick enough. Selintra thinks there is probably a setting in the ATA but I never bothered trying to find it. If you do please let me know
Regards,
Del