I have tried to use the driver with a Siemens C75 phone, connected via USB cable. Mac OS X 10.4.11, on MacBook Pro 2.4 GHz.
Not sure that this phone has a PL2303 chipset inside, but apparently the driver recognizes correctly the phone, since the corresponding device in /dev is created when the phone is plugged:
crw-rw-rw- 1 root wheel 11, 23 Jan 28 14:26 cu.PL2303-000023FA
Network Preference Panel detects a new port, called "PL2303-" (without the 8-hex extension). Configuration seems ok, but when dialing a number with Internet connect, the device seems to do not respond; eventually Internet Connect aborts with the generic error "A modem error occurred. Please verify your settings and try again". Nothing happens on the phone, in the meanwhile.
USB probe gives no information, during the connection attempt.
Any hint, at least for debugging?