Senior Software Engineer

Senior Software Engineer — Voice Relay

The mission

The telephone call is not dead — it's becoming intelligent. Voice Relay is Sinch's newest frontier: a real-time bridge that connects AI agents directly to live phone calls, handling speech recognition, voice synthesis, and interruption management so that developers don't have to. We're talking about replacing IVR hell with fluid, human feeling conversations — at global scale. We're looking for a Senior Software Engineer who is energised by hard infrastructure problems, proud of the systems they build, and excited about being at the intersection of real-time communications and the AI era. You'll be a core contributor to the team that owns, evolves, and scales Voice Relay — a product that is already in the hands of enterprise customers and growing fast.

What You'll Do

• Design and build the backend services that power Voice Relay — the real-time media pipeline, session orchestration, speech/TTS integration, and the developer facing API layer.

• Own your code end-to-end — from architecture review through to production deployment on Kubernetes in AWS and ongoing reliability.

• Tackle latency and scale together — real-time voice leaves no room for sluggishness; you'll instrument, profile, and optimize to keep every conversation sharp.

• Drive technical direction — contribute to architectural decisions, mentor peers, and raise the bar on code quality, testing, and observability practices.

• Work tightly with Product and AI teams — translate capabilities of LLM-based agents into reliable, production-grade infrastructure.

• Operate what you ship — participate in on-call, respond to incidents, and use what you learn to make the system more resilient.

Who You Are

You're a product-minded engineer who has a knack for understanding systems deeply — not just making things work, but understanding why they work and how they'll break. You care about reliability and performance the way others care about features. You thrive with autonomy and bring others along with you.

What you bring

• Substantial professional experience designing and running distributed backend systems in production.

• Deep expertise in C# and .NET Core — this is our primary stack on Voice Relay.

• A genuine DevOps mentality: you are comfortable with CI/CD, infrastructure-as-code, and consider observability a first-class concern.

• Strong communication skills and a collaborative instinct — you share knowledge freely and ask for help when it matters.

• A passion for clean, well-tested, maintainable code, and the conviction to hold that standard under pressure. We'll get especially excited if you have:

• Experience with VoIP and SIP — you know what a SIP dialog looks like and why RTP jitter matters. • Hands-on work with real-time media systems such as Asterisk, FreeSWITCH, or custom WebRTC/RTP stacks.

• Knowledge of speech technologies — STT/TTS pipelines, audio encoding, streaming APIs like those from Google, Azure, or ElevenLabs.

• Production experience with Kubernetes and AWS — you've been on-call for it, not just deployed to it.

• Exposure to LLM integration patterns — tool calling, streaming responses, turn taking logic in conversational AI.

• Background with relational and in-memory databases (MySQL, MSSQL, Redis).

• Experience in additional languages such as C++ — our ecosystem is polyglot.

About Sinch

Sinch's vision is to connect every business with every customer, everywhere in the world. We power over 900 billion customer interactions per year for more than 190,000 customers globally. Our Enterprise Voice platform — and Voice Relay as its latest innovation — puts us at the centre of how AI-powered businesses will communicate for the next decade. We're profitable, listed on Nasdaq Stockholm, and have over 4,000 employees spread across more than 60 countries.

"Salary

The monthly salary for this position is between 55 000-75 000 SEK. Factors which may affect pay within this range may include skills, education, experience, and other qualifications. "