This is a fine reason to use OSTicket or Request Tracker, both of which are free, open source and (with their various dependencies and modules) composed of entirely GPL, BSD and Apache licensed software.

Disregard Zendesk, acquire RT.

RT has been around for 15 years but it's continually developed and kept up to date. It's used by a large percentage of the 50 largest ISPs in the world for their back-end OSS/NOC ticketing.

Wait until they go after everything that's pronounced verbally like Zen. Most people I know if speaking about the Xen hypervisor will say "zen", not pronounce it as if it were a Chinese family name (Chen or how the X in Xiaoming is pronounced).