If you want to use a serial line as a dialin port, you have to enable a getty process on this port. However, some getty implementations aren't really suitable for this, because you usually want to use a serial port for dialing in and out. You therefore have to make sure to use a getty that is able to share the line with other programs like uucico, or minicom. One program that does this is uugetty from the getty_ps package. Most distributions have it; check for uugetty in your /sbin directory. Another program I am aware of is Gert Doering's mgetty, which also supports reception of facsimiles. You can also obtain the latest versions of these from sunsite.unc.edu as either binary or source.
Explaining the differences in the way uugetty and mgetty handle logins is beyond the scope of this little section; for more information, please refer to the Serial HOWTO by Grag Hankins, as well as the documentation that comes along with getty_ps and mgetty.