If you are on Solaris 2.x, You should use pkgadd to add the vendor supplied package. If you write your own drivers, you have to add it into
/kernel/drv directory, and provide .conf file if necessary. Also in the init.d
directory, you need to add necessary scripts to facilitate driver's probing and initialization(for instance, use "add_drv" to create device file,etc)
If you are on Linux platform, use modins to install driver modules(driver module is a kind of driver that can be dynamically load in kernel and linked with kernel symbol table).

depending on what UNIX and whether the Modem is popular one or NOT. if the Modem is popular products, the UNIX usually has its driver configED in kernel. otherwise you have to install the Modem's driver. check your DSF(device special file, /dev/tty00, /dev/tty01, ) make sure has NOT been occupied.
also, you need to think about outgoing connection or bi-direction connection of Modem. so, maybe you have to add a line into /dev/inittab file. at last , edit two file for baut rate, couterpart host name, modem commands, etc., check the feature of /dev/tty* (maybe you have to use shell command " stty " to change the feature ) . good luck