B2BUA (Back-to-Back User Agent) is a powerful VoIP server component that controls and manages calls by acting as both a client and a server. It sits between two endpoints, establishing separate call legs and relaying information between them. Unlike a proxy, B2BUA has full control over call signaling and media, enabling advanced features like call recording, billing, and security. It also enhances privacy by masking user details. B2BUA is crucial for complex VoIP deployments, including call centers and enterprise communication systems.

