No modem!

I just installed Mandrake 7.1 with the PPP service. I set up pppd, but it tells me the modem is busy when I try to dial. I’m using an external modem on the first serial port (cua0/ttyS0) but I never see any lights toggle, even when booting into Linux. The same hardware multi-boots into WIN95 and
NT 4.0 and I can use the modem with no problems. Running one of the hardware scanners under Gnome or KDE shows no modem installed.
What’s next?