General info on LAN setup please

[pointdee]

I'm a little confused as to what equipment I would or wouldn't need if I wanted to implement some form of IP telephony on our local LAN. The idea initially would be to use this internally only and for external calls to go through our current ISDN line. Apparently there is a card available for our exchange to do this.

Our LAN uses dumb switches (not managed) and wireless access points to cover the entire site but all rooms have at least one wired network connection.

Is this existing infrastructure enough or would we need switch upgrades (IPT switches)? Presumably we would need a server to manage the address to phone number conversion but is there anything else?

All comments, links to general info for a beginner etc gratefully accepted.

Thanks