[PPP] serial cable-modem recognized, but not responding

[H-Pi]

I try to get my internet connection working, using a serial cable-modem, reading the PPP gentoo howto in documentation & tricks

when I do "wvdialconf .etc.wvdial.conf", wvdial recognizes the modem and speed etc., but a "wvdial" returns:
--> WvDial: Internet dialer version 1.53
--> Initializing modem.
--> Sending: ATZ
--> Sending: ATQ0
--> Re-Sending: ATZ
--> Modem not responding.

that is, the same messages when there is no modem plugged in

somebody knows how to deal with this?