H323 is a protocol, similar to SIP, designed for audio and visual communication over an IP network (VoIP, Internet Telephony, or IP Telephony) and for IP-based videoconferencing.
This set of ITU standards (umbrella recommendation) for multimedia communications, which refer to terminals, equipment, and services that establish signaling on IP networks, does not guarantee quality of service, and data transport may or may not be reliable. In turn, it is independent of the network topology and supports gateways, allowing more than one channel of each type (voice, video, data) to be used at the same time.
Because of these features, Voice uses SIP, so if you're using H323 and need to implement Voice you'll need to find a way to integrate SIP with your telephony provider.