Wellll, if you wanted to be really cheap, you could install an analogue line to a PC with a voice modem, and plug a headset into the PC or modem.
There's no shortage of software out there that will accomodate your request, such as TalkWorks.
Having said that, the voice quality is not going to be that great, and you'll most likely be troubleshooting PC problems than anything else.
Alternatively, you could install Skype and have going out through the data network, or the voice network, though you'll need Skype hardware to connect it to the PABX.
Skype however, tends to give the network security guys the willies, as it has a similar network fingerprint of a worm.
So, certainly not as expensive as the Nortel offerings, but each come with their own caveats.
Good judgement comes from experience, which usually comes from bad judgement.. [i/]