Retell is building the new standard for automating call center communications, including sales, support, customer engagement, and retention calls.

Backed by Altman Capital, Y Combinator, and top investors, we've raised a $4.7M seed round and hit $7M ARR in 12 months.

Check us out on the Top Lean AI Native Companies Leaderboard at leanaileaderboard.com.

Retell AI - API That Turns Your LLM Into A Human-Like Voice Agent

Retell AI Basics: Everything You Need to Start Building Voice Agents

Vision

Twenty years ago, every business found itself needing a website, the new gateway to the world. Platforms like WordPress and Squarespace rose to become the standard for getting businesses online.

Today, a new revolution is here. Every business that relies on phone interactions can now have an AI voice agent of their dreams. A tireless and empathetic receptionist, salesperson, debt collector, or appointment reminder that works 24/7. Operational bottlenecks vanish overnight. Doubling your business no longer means doubling your team. Customer experience no longer suffers. (Ever tried calling an East Coast bank after 3 p.m. PT?) But companies aren't ready. Platforms don't exist.

At Retell, we're pioneering the new standard for building AI voice agents. With seamless UI, comprehensive toolkit, effortless integrations, thriving builder community, we're creating everything businesses need to deliver exceptional phone interactions on every single call.

Join us—because the future of AI voice agents is coming anyway, so why not be the one who defines it?

Role Overview

As a Founding Senior Machine Learning Engineer at Retell, you’ll be responsible for developing and deploying the core AI models that power our voice agents—from speech recognition and synthesis to natural language understanding and prompt orchestration. You’ll work hands-on across the ML stack, fine-tuning state-of-the-art models, creating robust evaluation pipelines, and ensuring they perform reliably in real-time production environments. This is a high-ownership role where your work directly impacts millions of conversations, and you'll have the opportunity to shape Retell’s AI strategy alongside the founding team.

Position: Founding Senior Machine Learning Engineer

Job type: Full-time 70hr/week (50 hr/week onsite with flexible hours + 20 hr/week work from home)

Salary: $200K - $310K

Equity: 0.50% - 1.0%

Bonus: $20k - $100K

Location: San Carlos, CA, US

US visas: Sponsors Visa & Greencard

Benefits: 100% medical, dental, vision insurance covereage. Unlimited breakfast, lunch, dinner, and snacks. Gym, daily communte fee reimbursed. Internet, phone bill covered.

Job Description

Roles and responsibilities

• Create datasets, benchmark models, and define evaluation metrics for various tasks in voice agent platform.
• Train / Fine-tune various audio & LLM models for accuracy, latency, and domain specific knowledge.
• Work with engineers to integrate, monitor, and optimize models in production.
• Setup and manage a scalable human evaluation pipeline.
• Stay updated on cutting-edge AI advancements and experiment with / benchmark new models.
• Create scalable ML infrastructure & deployment within the system.

Qualifications

• 2+ years of experience in machine learning, LLM and audio modeling experience is a plus.
• Master’s degree or equivalent practical experience in AI / ML.
• Experience in improving, integrating, and deploying models in production systems.
• Proficient in Python, Pytorch, and other ML frameworks and tools.
• A founder-like mindset with a passion for taking responsibility and making a significant impact.
• Ability to thrive in a fast-paced environment, quickly learning and adapting to new challenges. Previous startup experience is a plus.
• Excellent communication and collaboration skills, with the ability to commit to decisions even when there are disagreements.

Benefits

• Competitive salary with an equity package, year end bonus
• Best health, dental and vision insurance available, 100% covered by Retell for you
• Catered breakfast, lunch and dinner everyday
• Monthly commute, gym expenses fully covered
• Unlimited snacks and drinks
• Monthly Wellness reimbursement that you can spend on anything health related
• H1B and Green Card sponsorship available
• The chance to innovate and push the boundaries of voice AI in a dynamic, venture-backed startup

Interview Process

Online Assessment

The online assessment will be sent through HackerRank. It consists of one coding question and takes approximately 25 - 30 minutes to complete. Candidates have 7 days to submit their responses.

Phone Interview 1

A 30 minute interview focused on ML coding.

We will share a pair coding IDE link (Coderpad) and use Python w/ ML packages.

Phone Interview 2

A second 30-45 minute interview on data structures and algorithms.

We will share a pair coding IDE link (Coderpad) and you can use your preferred programming language.

Onsite

This step involves 1-4 hours of interviews, onsite is in the San Francisco Bay Area. If the candidate lives outside of the bay area, it may be conducted virtually.

Please bring a computer for writing code during the interviews.

There are up to 4 interview rounds:

• A ML fundamentals round where we will go through ML foundations, deep dive into your past ML projects.
• A data structures and algorithms coding session. We will share a pair coding IDE link (Coderpad) and you can use your preferred programming language.
• A ML system design interview, which tests the candidate’s ability to transform a real life problem into scalable ML solutions.
• A behavioral interview covering past work experience and work style.