Asterisk "That is not a valid conference number, please try again"

Problem: you've configured a conference call room with the MeetMe application in Asterisk, but when you call it, a voice says "That is not a valid conference number, please try again".
For some reason, asterisk needs a zaptel device, or a dummy driver to use MeetMe. If you don't have zaptel hardware, you'll need to use the ztdummy driver (this might be renamed in new asterisk releases). If you're using Debian 4.0, run "aptitude install zaptel-source", followed by "m-a prepare", "m-a build zaptel", "m-a install zaptel", "modprobe ztdummy". Call you're conference room number, it should work now :)