Modem doesn't respond

[Pee]

I have an internal 56k v90 USRobotic fax modem connected in Windows to COM2 and in Linux to ttyS1. It is working in Windows, but in Linux I always get a message saying that the modem does not respond. Do you know how to solve this problem? I tried on RedHat 6.1, Corel Linux, and Debian 2.1 and I always get the same message. TIA - Pee