Voice agents are transitioning from simple FAQ responders to systems capable of performing complex actions, raising the challenge of verifying caller identity without traditional visual authentication methods. This evolution necessitates robust, voice-based authentication systems comparable to human-operated call centers to securely manage sensitive transactions. The article discusses several authentication strategies, including session-based methods, telephony-specific techniques, and the use of one-time codes, all designed to ensure deterministic authentication without relying on the judgment of large language models (LLMs). These methods emphasize the importance of tool-based verification and workflow gating on platforms like ElevenLabs to isolate authenticated users and prevent unauthorized access. The implementation of these strategies involves integrating with CRMs and utilizing dynamic variables for seamless user verification, while also considering security trade-offs such as rate limiting and multi-factor authentication to address potential vulnerabilities. Ultimately, the choice of authentication approach should align with the organization's risk profile and regulatory requirements, allowing for an adaptable security framework that balances user experience with protection.