Retell.ai vs Sierra AI Comparison (2025)

Retell AI offers fast, scalable voice agents with response times under a second and impressive developer flexibility. You can bring your own LLM, customize behavior deeply, and monitor everything through real-time dashboards. It’s well-suited for teams with in-house engineering that want control and scale. But the learning curve is real—getting up and running takes work, integrations often require custom scripts, and pricing isn’t transparent. While it performs well under the hood, Retell isn’t the most approachable platform for teams seeking simplicity or industry-ready templates out of the box.

Sierra AI delivers a comprehensive conversational platform with multi-channel voice and chat, real-time analytics, and enterprise-grade security out of the box. But you won’t find a price list anywhere—expect to negotiate usage-based fees that climb steeply as you add advanced features or extra channels. Its “build once, run everywhere” Agent OS sounds neat, but customizing flows and integrating with legacy systems often requires dedicated engineering time. The command center offers deep controls, yet teams frequently find it overengineered for routine tasks and in need of training to manage. And despite claims of empathetic, human-like responses, at scale the agents can still stumble over accents or complex queries, leaving gaps that demand human intervention.