Having worked for AOL as the name suggests I've had alot of experience of this issue. Many people claim that ISP's tell them that software modems or out of date drivers cause issues and that this is a lie.
To be honest this could be your issue. The trouble is ISP's just dont have time to test their software with various types of modems and brands. This is why issue arise as support is only really given for popular brands. My advice is to go out and get yourself an external 56K modem or a decent internal one. You could try the latest drivers for your software modem.