There are lot of guides about installing FreeSwitch on Debian/Ubuntu. The official guide is at http://wiki.freeswitch.org/wiki/Installation_Guide#Debian_Linux and another is at howtoforge. Ubuntu has deb packages even and we may use apt-get install (so easy). Debian lenny does not have bin packages. So we will install the freeswitch from the source.
Installation
First, add repository to system
vim /etc/apt/sources.list
add line