Voice over Internet Protocol (VoIP) systems provide flexible and cost-efficient communication, but their dependence on Internet infrastructure exposes them to identity theft, session hijacking, replay attacks, call tampering, and denial-of-service attacks. Conventional VoIP authentication mechanisms generally rely on centralized servers and password-based credentials, creating single points of failure and attractive targets for attackers. This paper proposes a blockchain-based authentication framework for secure VoIP communication. The framework uses a permissioned blockchain to manage decentralized identities, public-key credentials, device registration, and authentication events. Smart contracts enforce registration and credential-revocation policies, while cryptographic challenge-response protocols provide mutual authentication between users and VoIP endpoints. The proposed design separates sensitive voice media from blockchain transactions, thereby preserving the latency requirements of real-time communication. Security analysis indicates that the framework can improve resistance to credential forgery, replay attacks, unauthorized endpoint registration, and centralized database compromise. The paper also discusses performance, privacy, scalability, and deployment challenges associated with blockchain-enabled VoIP authentication.