When is H.323 or SIP used? Is it used just for IP telephony (IP phones and gatekeeper/call managers) or also with VoIP (I.E. regular phones to routers to WAN)?
H.323 and SIP are often compared and do compete with each other for VoIP services. H.323 has been the early leader in this market so it is very popular. There are standards groups who view the Session Initiation Protocol (SIP) as the way to go in the future but sometimes the early leader ends up winning anyways. H.323 is still very important to product development and time will tell which one survives or whether both survive or remain important.
Here is a pretty good comparison of the two: http://www.packetizer.com/iptel/h323_vs_sip/