sbiddle: You can use a Cisco but you need to convert the phone from Skinny (Cisco's proprietory protocol) to SIP which is the industry standard protocol used by most devices these days. You have to buy a licence from Cisco to do this but it shouldn't take too long if Google is your friend to find out how to do it.

They have a very complicated licencing system, but basically you need a licence for each phone to use Cisco CallManager.

The instruction to convert a 79xx to SIP are here

http://cisco.com/en/US/tech/tk652/tk701/technologies_tech_note09186a0080094584.shtml

You will have to obtain the firmware however as it's not publically available from the Cisco site. Unless you really want to spend a lot of time playing about I wouldn't recommend trying to get one of these phones working on iTalk. They are one of the best IP phones available but unless you have time to kill and want to learn a few things along the way buying a SIP compatible phone or an ATA like the Linksys PAP2 which allows you to use normal analogue phones will be a lot easier!