Update: I was able to kludge and tweak Motorola's binary-only driver (that's now unsupported) to work with my Mandrake 8.1 installation. The modem now dials out and performs handshaking, but it won't connect (yet). Keep you people posted about any news, as well as complete details.
Technical details:
Modem chipset: (internal) Motorola SM56 PCI I/II.
Driver: released by Motorola, RPM for Red Hat 7.1 or kernel 2.4.6(?); is not tested for other distros or kernel versions.
Linux installation: Mandrake 8.1 (kernel 2.4.8-26mdk)
Tweaks: to follow ;)
Previously: No Subject