UI_character_server.INI in the Everquest directory. Find ChannelAutoJoin=whatever and delete it.
You can also use in-game command to re-assign channels, if you actually different ones, rather than none. It is: /autojoin channel1:password1,channel2:password2,...,channelN:passwordN
where :password# is optional.
Some channels are auto-joined under Options also.
Yther Ore.