H.323 is, much like SIP, a protocol designed for the setup, management, and termination of a media session. It is one of a set of standards from the ITU-T, which defines a large set of protocols to provide audio and visual communication over a computer network.
H.323, like SIP, is a relatively old protocol but has been largely superseded by SIP (Session Initiation Protocol). One of the advantages of SIP is that it’s much less complex and resembles the HTTP/SMTP protocols. In this respect, H.323 is a binary protocol, making it less technician-friendly in a troubleshooting environment.
H.323 was not designed for easy extensibility, with the result being that new features require more time to be defined, standardized, and implemented. SIP, on the other hand, is designed from the ground up to be extensible, and a SIP entity that receives a message containing features that it does not recognize can simply ignore the features. SIP is therefore much better able to keep up with the current market and technical needs of the IP telephony world.
We can see how H.323 is falling into disuse by looking at endpoint devices (phones in particular). A few years ago, the previous trend of creating devices that could use both protocols was abandoned by many phone manufacturers. The vast majority of devices available on the market today are SIP-only.
The result is that most VoIP equipment available today, particularly including IP PBXs, only follow the SIP standard.