About Retell AI
At Retell AI, we're not just automating calls—we’re transforming how the world communicates. Our AI voice agents are reshaping sales, support, and customer engagement for leading brands. Backed by Alt Capital, Y Combinator, and top-tier investors, we've raised $4.7M in seed funding and hit $14M ARR with just 12 people.
We’re one of the fastest-growing Voice AI startups and we're on a mission to become the standard for voice automation at scale. We're also one of the top-ranking startups at leanaileaderboard.com.
About the Role
As a Founding Senior Telephony Engineer, you’ll own the foundation of our telephony stack, building the critical systems that connect our AI voice agents to the real world. From low-level SIP features to integrating with global carriers and CCaaS platforms, your work will directly impact call quality, reliability, and customer experience. You’ll move fast, collaborate with customers, and help define the gold standard for AI voice infrastructure.
Key Responsibilities
Lead design and implementation of core telephony features, including SIP server enhancements and proxy logic.
Build and extend our SIP infrastructure to support scalable, low-latency audio routing and call control.
Collaborate with customers to support complex SIP integrations with CCaaS platforms and third-party providers.
Evaluate and integrate with global telephony carriers to optimize deliverability, latency, and transfer logic.
Investigate and debug call failures via PCAP analysis, SIP traces, and carrier logs to improve reliability.
Work cross-functionally with platform and infra teams to ensure telephony systems scale with usage growth.
Turn feedback from customer calls into robust and maintainable telephony improvements.
You Might Thrive If You
Have 5+ years as a Telephony Engineer, VoIP Engineer, or equivalent low-latency networking role.
Have deep experience with SIP, RTP, and debugging voice quality issues with PCAPs and trace tools.
Have built or extended SIP proxies, PBXs, or call routing logic in production systems.
Thrive in fast-paced, ambiguous environments and take ownership like a founder.
Communicate clearly, collaborate well, and enjoy solving real-world problems with users.
Familiarity with Asterisk/FreeSWITCH/Kamailio or relevant SIP proxy / SIP servers.
Job Details
Job Type: Full-time, 70 hr/week (50 hr/week onsite with flexible hours + 20 hr/week work from home)
Cash: $215k–$290k
Equity: 0.3%–0.6%
Location: Redwood City, CA, US
US Visas: Sponsors Visa & Green Card
Other Benefits
100% medical, dental, vision insurance coverage
Unlimited breakfast, lunch, dinner, and snacks
Gym and daily commute fee reimbursement
Internet and phone bills covered
Compensation Philosophy
Best Offer Upfront: Choose from three cash-equity balance options; no negotiation needed.
Top 1% Talent: Above-market pay (top 5 percentile) to attract exceptional builders.
High Ownership: Small teams, >$1M revenue/employee, and significant equity.
Performance-Based: Offers tied to interview performance, not experience or past salaries.
Interview Process
Online Assessment (30 min)
A short coding challenge focused on practical problem-solving.
Phone Interviews
Two technical phone calls focused on coding ability and telephony problem-solving.
Onsite / Virtual Interviews (2–3 hrs)
A mix of coding, communication, and system design discussions to evaluate technical depth, real-world problem-solving, and collaborative thinking.
