I had the same problem with my DEC HiNote VP765 and Xircom RBEM56G. I was using Red Hat 6.1 and kernel 2.2.13 and pcmcia 3.1.1. The network side of the card worked fine but trying to use minicom to even get an OK from the modem was impossible. Minicom claimed the modem on online even after a forced hang up.
I'm suspecting something in the 6.1 release is the problem because I'm using the same kernel now with Red Hat 6.0 and it works fine. I also tried an external modem on the rear serial port and it showed the same behavior. So I guess it can't be blamed on the pcmcia stuff.