Having bought a cheap voice over IP phone (Budgetone 101, sell for around £50 on eBay) I decided to stick Asterisk on the Debian box. What a fantastic bit of software! Asterisk works on the principle that when a phone call has been routed to a computer, you can do pretty much anything you like with it. By editing some flexible configuration files you can do simple things such as taking messages and call forwarding or more complex tasks such as redirecting calls to internal phones, DTMF menus, etc.
Asterisk now has a permanent home on my server, where it sits forwarding incoming calls to my Sipgate account to my mobile phone, for just 14p a minute, rather than the extortionate prices that UK telcos like to charge. If you want to explore Asterisk VoIP User has a useful forum and VoIP Info’s wiki also contains some good information.