Customizable, no-code voice agent automation with GPT-4o

Retell AI⁠(opens in a new window) is rebuilding the call center from first principles. Instead of retrofitting legacy call scripting systems, they’ve created an AI-native, no-code platform that spins up natural-sounding voice agents that can answer questions, schedule appointments, and resolve administrative issues—no hold music, scripting, or heavy engineering required. 

This category of technology, known as voice automation, enables businesses to handle calls instantly, around the clock, while eliminating hold times, reducing operational costs, and improving customer satisfaction.

By closely tracking OpenAI model releases and integrating them quickly, Retell can deliver new capabilities to customers within days. As co-founder and CTO Zexia Zhang explains, “Because the models keep getting better, our platform keeps getting better. We’re not just scaling faster, we’re tackling more complex problems with a simpler stack.”

That approach is paying off: teams using Retell report up to 80% reductions in call handling costs, and Retell is seeing a high NPS of 90 across deployments. 

Unlocking real-time, human-like conversations with GPT‑4o 

To enable natural conversations, Retell needed an AI provider that could move beyond scripted exchanges, something that could understand context, follow logic, and trigger the right actions in real-time. After evaluating multiple LLMs, OpenAI stood out for its precision and performance. 

“OpenAI’s models strike the best balance of speed, reliability, and intelligence,” says Zhang. “When we tested function calling, GPT models were not only the most accurate, they were the fastest to respond. That level of performance is critical for real-world conversations.”

Retell voice agents use function calling to schedule appointments, qualify leads, and resolve administrative issues. In each case, the model needs to reason across multiple turns of conversation and invoke the right API at the right time.