The real work-horses are actually Kamailio and Sems, which are quite little known but power a LOT of your ISPs' VoIP systems.

Provisioning is built on top of Apache/Perl/Catalyst with a MySQL backend. The billing system is in C and Perl, and the Media Relay is in C with an own kernel module on top of iptables.

Asterisk is pretty insignificant, but it's surely the best known part in the VoIP world.