Senior Machine Learning Engineer

ABOUT RETELL AI

Retell AI is using the first principles to reimagine the call center with cutting edge voice AI.

We believe voice is still the most natural way humans communicate, yet it has been trapped in outdated call centers for decades. Our mission is to bring intelligence, empathy, and speed to every phone conversation between businesses and their customers.

Since launching 18 months ago, thousands of companies now utilize Retell’s AI voice agents to handle sales, support, and logistics calls that once required large teams of human agents. Backed by Y Combinator, Altman Capital, and other leading investors, we have scaled to $30M ARR with a team of 20 people, up from $5M at the start of 2025.

Now, we’re scaling fast, and we’re looking for bold, ambitious people to help us build the gold standard for voice automation. If you want to work on deeply technical challenges, move fast, and make an outsized impact at one of the fastest‑growing Voice AI startups in the world, you’ll love it here.

Let’s build the future of voice automation together.

• We’re a top 50 AI app in a16z list:

• We're also one of the top ranking startups on:

• Build the Dream Team: We hire high performers who work onsite in the San Francisco Bay Area, move fast, and collaborate deeply. Feedback is open, direct, and constant.

• Customer obsession. We start with the customer and work backwards. Every job and task matters only if it ultimately benefits the customer.

• Insist on the Highest Standards: We ship products you’re proud to show everyone — even if it means spending more time perfecting them.

We like to keep it real, we move fast, build fast, and change fast. That’s part of the fun for us, but we know it’s not for everyone.

This is a hands‑on, high‑ownership role for ML engineers who want to build production models that actually ship, and perform under real‑world constraints. As a Founding Senior Machine Learning Engineer at Retell, you’ll work across the ML stack to power human‑like voice agents that handle millions of real‑time phone conversations.

You’ll fine‑tune large language models and audio models, evaluate them with rigorous benchmarks (and human feedback), and deploy them into latency‑sensitive, high‑traffic systems. You’ll own model performance end‑to‑end—from training pipelines to post‑deployment monitoring—and shape our ML strategy alongside the founding team.

If you’re excited by hard technical challenges, fast iteration, and the opportunity to define how voice AI works at scale, this role is a rare chance to do it from the ground up.

• Train & Tune Models – Fine‑tune LLMs and audio models to maximize speed, accuracy, and production‑readiness—pushing the frontier of real‑time AI voice experiences.

• Benchmark & Evaluate – Build datasets, define rigorous metrics, and measure model performance across high‑impact voice AI tasks to guide development.

• Deploy to Production – Work closely with engineering to ship models, monitor them in the wild, and ensure they stay fast, reliable, and accurate at scale.

• Run Human Evaluations – Build scalable pipelines to collect structured human feedback, benchmark subjective quality, and inform model iterations.

• Level Up Infrastructure – Design and maintain the ML infrastructure needed for fast experimentation, robust training, and continuous deployment.

• ML Engineer with Real‑World Experience – You’ve trained and shipped models in production. Bonus if you’ve worked with LLMs or audio models.

• Fluent in Modern ML Stack – You know your way around Python, PyTorch, and today’s ML tools—from training pipelines to evaluation benchmarks.

• Execution‑Oriented – You move fast, take ownership, and focus on solving real problems over perfect ones.

• Startup‑Ready – You’re adaptable, resilient, and energized by ambiguity and fast‑changing priorities.

• Clear Communicator & Team Player – You collaborate well across functions and push decisions forward.

• Cash: $200,000 - $280,000 base salary

• Equity: 0.15% – 0.35%

• Location:
  
  Redwood City, CA, US

• US Visas:
  Retell AI is open to sponsoring work authorization for qualified candidates, including H1B/H-1B, TN,…