The two major protocols used for VoIP are SIP and H.323. (They are completely different standards, not related to each other). SIP isn't so much a complete VoIP protocol - other protocols are required for the actual voice data transmission. See the SIP page for more details.
Cisco also deploy MGCP and Skinny? on their AVVID solution.
See 
http://voip.fast.co.nz , a NZ voip PABX you can connect to and make free calls.
This client/server works for both Win32/Linux. It's a great PC to PC communication program.
http://www.zultys.co.nz/LIPZ4.htm http://www.zultys.com/products/lipz4/softphone-1.3.11-0.i386.rpmKphoneLinphone - Free (GPLd) and GTK2. (as of Sep 2004 this is now in Debian Sarge and Sid (unstable))SJPhone SJ Labs VOIP softwarehttp://www.xten.com/index.php?menu=products&smenu=xlitehttp://www.virbiage.com/firefly/Approved FWD Cisco SIP Clients
Example of a SIP gateway using a Cisco ATA-186 connected through a small PBX
http://www.openh323.org/
GnuGK and friends make up the openh323 project. GK is an h.323 gatekeeper (central server thing).
A more 'hardcore' SIP server. Less features, more commercial. SIP supports textual messaging, so SER has a Jabber gateway.
Very easy to use, free for personal use and actually works quite well (And Windows versions also) Alas, no presence (i.e. no IM (sigh))
http://www.microsoft.com/office/livecomm/prodinfo/default.mspx
SIP, H.323, etc. Nice and intergrated.
E164 (E164.org) http://www.e164.org/
Clients can be connected to more than one service at a time, think of it as having 2 separate phone lines, both connected to different providers but both phones plugging into one unit.. so feel free to sign up for accounts at more than one service.. you can select which service you want when dialling away from the default service by starting to dial your numbers with #2 for the second account you have setup etc.
SIP is the new protocol used to initiate communication between various units, and it is a standard met by clients on Linux, Windows, or on stand-alone VoIP phones. This means you can call any user on the network not caring what kind of hardware they are using, the SIP should help start a common conversation so you can talk to each other.
To try it out, get yourself a client from the links listed at the top of this page, if you find any other clients please add them to this list, or if you want to point out problems or good points add them to this list also. Then register with a service listed on the SipServices page.
If you have a public IP address with the right ports unfirewalled, people can also connect directly using asip://username@ip.add.re.ss URI.
http://www.digium.com - This crowd produce a PCI PSTN gateway card, no idea at this point as to wether it works in New Zealand. Of more interest however is their PBX gateway software belowhttp://www.asterisk.org - This is an Opensource PBX gateway that supports VoIP <-> PSTN mappings as well as SIP, H.323 and a rather large number of codecs.http://www.pingtel.com - A VoIP handset phonehttp://www.snom.com - Another VoIP handset, this site is great tho, lots of VoIP resources. AND their phones use linux as the base OS !http://www.sipcenter.com/ - Everything you didn't want to know about SIP (Session Initiation Protocol)From the looks of things, the Quicknet LineJACK cards also work in New Zealand - they are currently being deployed by a crowd in christchurch, Telepermiting of these cards is unknown at this stage (waiting for a reply from the telepermit guys)
5 pages link to VoiceOverIP:
