It should be in the wiki top right of this page or gentoo forums / wiki.
kppp needs to be installed if not already then make sure using lspci to see if the modem is listed , if not "modprobe greenelephants" replacing greenelephants with correct module / driver for modem then on to kppp or similar.
Have not done dail-up yet so not actual clue but this might get you started.
# net-setup should work as well once modem has a driver / module for it and or working.
I had some success with a "web excel" connexant 56K 'data/fax/voice' external modem on com1 [/dev/tts0]. I found I had to find the modem I think in ^system ^hardware. Then configure it in kppp. Though I found that kppp's ^querie modem couln't always find it ^connect seemed to connect ok.It later appeared in ^querie modem. Also got an error message from kppp to enter as root the command "mknod /dev/ppp c 108 0" or the connection wouldn't hold, I seem to have to type this in every time for a new session.I've only been using this distro for several days & aren't very experienced with linux. Also found that dialup speed is at >1.3kB/s. I have to use XP for my weird One way satellite broadband. Good luck,hope this helps