To build an H.323 Trunk in Avaya Aura Communication Manager, three crucial elements must be configured. An ISDN Trunk Group is required as it refers to the type of trunk used for H.323. An H.323 Signaling Group is necessary to manage the signaling for the H.323 trunk. Additionally, a Name/IP-Address entry must be added to the Node-Names IP table to associate the H.323 entities with their respective IP addresses, which is essential for establishing communication.