Staff Software Engineer — AI Communications Platform

Company: 21X formerly Structurely Location: Detroit, MI / Remote Employment Type: Full-Time Seniority: Staff / Principal Engineer About 21X21X formerly known as Structurely has been building the next generation of AI-powered communications for over 10 years and is a 2026 winner in top conversational AI platforms for 2026 ranked by Twilio 👉(https://www.twilio.com/en-us/blog/insights/conversational-ai-platforms)Over the last decade, we have been developing conversational AI technology that helps businesses engage customers, automate conversations, drives systems and measurable business outcomes. Today, our unified AI communications platform powers AI voice, SMS, RCS, email, and other emerging communication channels. Our platform has primarily served enterprise customers across mortgage, real estate, home services, and banking. And now, we have PMF in private aviation, sports, and hospitality. We want to win. Period. We’re a lean, highly competent, ambitious organization who take risks and tackle complex technical problems at enterprise scale. We want engineers who want to win, have agency, move quickly, and love building cool things.

The Opportunity: We’re looking for an exceptional Staff Software Engineer to own the architecture, development, and evolution of our AI communications infrastructure to work alongside our executive leadership team. This is not a traditional telecom engineering position and we are not a traditional team. You’ll work at the intersection of real-time communications, telecommunications infrastructure, modern messaging protocols, and artificial intelligence. You’ll help build the underlying systems that allow AI agents to communicate with humans naturally, reliably, and intelligently across multiple channels. From optimizing real-time voice quality over telephone networks to building rich, interactive RCS messaging experiences, you’ll solve some of the most interesting engineering challenges in conversational AI.This is a highly technical, hands-on role with significant ownership and architectural influence. We want a builder, not someone who just manages builders. What You’ll Own1. Real-Time AI Voice Infrastructure Architect, build, and optimize our real-time AI voice communications stack. Develop and maintain SIP, VoIP, WebRTC, and telephony infrastructure. Integrate and optimize carrier connectivity across Vonage, Twilio, and other providers. Build high-performance, low-latency audio streaming pipelines. Optimize speech-to-text (STT), text-to-speech (TTS), and AI model integrations. Solve complex audio engineering challenges involving codecs, transcoding, jitter, packet loss, and network latency. Improve voice quality and conversational responsiveness over traditional telephone networks. Develop sophisticated call routing, transfers, answering machine detection, and failover capabilities. Build for high concurrency, reliability, and enterprise-scale traffic.2. RCS & Next-Generation Messaging Infrastructure Lead the technical development of our next-generation messaging capabilities. Architect and implement RCS Business Messaging (RBM) infrastructure. Develop integrations with messaging providers, including Vonage and Google’s RCS ecosystem. Build support for rich cards, carousels, interactive buttons, suggested replies, and rich media. Design intelligent SMS/MMS fallback mechanisms. Develop reliable message delivery, routing, and event-processing infrastructure. Implement messaging identity, sender verification, and compliance capabilities. Optimize messaging performance, deliverability, and cost. Help evolve RCS into a highly interactive, application-like experience inside native messaging environments.3. Cross-Channel AI Communications Build the communications layer enabling AI agents to interact with customers across channels. Architect unified communications services supporting voice, SMS, RCS, and future channels. Develop event-driven communication workflows. Enable seamless transitions between voice and messaging. Design systems supporting persistent conversation state and session continuity. Build communication APIs that integrate with our AI agent orchestration platform. Collaborate with AI engineers on real-time agent interactions, tool execution, and workflow management. Ensure consistency, reliability, and observability across communication channels.4. Enterprise Infrastructure, Reliability & ScaleBuild infrastructure capable of supporting millions of AI-powered interactions. Design scalable, fault-tolerant communications systems. Build high-concurrency voice and messaging infrastructure. Implement carrier redundancy, intelligent routing, and failover. Improve latency, throughput, uptime, and overall system performance. Develop monitoring, logging, tracing, and observability capabilities. Optimize infrastructure and carrier costs at scale. Ensure secure, compliant, and reliable handling of enterprise communications. Establish engineering standards for performance, testing, and production reliability.5. Developer Platform & Communications APIsHelp build the foundational infrastructure behind the 21X Developer Platform. Design and develop APIs for voice, SMS, RCS, and communication events. Build developer-friendly interfaces for integrating AI communications into external applications. Implement authentication, authorization, and tenant isolation. Develop scalable webhook and event-streaming infrastructure. Support usage tracking, metering, and billing capabilities. Build reusable communications services that can be leveraged by both internal teams and external developers. Collaborate on SDKs, technical documentation, and developer experience.

What We’re Looking For: Required Experience 8+ years of professional software engineering experience, with substantial experience designing and operating distributed systems. Deep experience building production-grade real-time communications, telecommunications, or messaging infrastructure. Strong understanding of SIP, VoIP, WebRTC, and modern telephony architectures. Experience integrating telecommunications and voice providers such as Twilio, Vonage, Telnyx, Bandwidth, Vapi, Deepgram, Sonniex or similar platforms. Strong proficiency in TypeScript/Node.js, Go, Python, Rust, or comparable backend technologies. Experience designing high-throughput, event-driven distributed systems. Strong knowledge of asynchronous processing, message queues, APIs, and real-time communication patterns. Experience working with cloud infrastructure, preferably AWS.Demonstrated ability to architect, implement, deploy, and operate complex production systems. Strong debugging and performance optimization skills. Ability to lead technical decisions while remaining deeply involved in writing production code.

Highly Preferred: Direct experience with RCS Business Messaging (RBM), Google’s RCS ecosystem, or carrier-grade messaging infrastructure. Experience developing voice AI systems or real-time conversational AI platforms. Experience integrating streaming STT, TTS, and large language models. Familiarity with Vapi, Deepgram, Sonniex, ElevenLabs, Cartesia, Fish, Experience optimizing real-time audio over PSTN networks. Knowledge of audio codecs, RTP, media streaming, and telephony signal processing. Experience building multi-tenant communications platforms or developer-facing APIs. Familiarity with A2P messaging, sender registration, and telecommunications compliance. Experience working with carrier networks, CPaaS providers, or messaging aggregators.

Why Join: 21X?Build at the frontier of AI and telecommunications. We’re bringing together real-time voice, advanced messaging, and AI agents to fundamentally change how businesses communicate. Own meaningful technology. You’ll have significant architectural ownership and direct influence over our core platform. Solve real engineering problems. This isn’t another wrapper around an LLM. We’re building and optimizing the communications systems that make enterprise AI work in the real world. Well Funded: We are a 10 year old startup, but we have capital and cashflow and are investing BIG in our future. Hockey Stick Growth Our technology has booned. Today we operate across many verticals, but our core verticals are home (mortgage, real estate, insurance, home services), aviation, and sports. Help shape the next generation of communications. AI agents will become a fundamental part of how businesses interact with their customers. We’re building the platform to make that possible. The Bottom LineWe’re looking for an exceptional engineer who have built modern AI communications systems and wants to help build what comes next. Someone who can architect a real-time voice platform, solve complex telecommunications challenges, build next-generation AI Voice, RCS messaging, and create infrastructure that allows AI agents to communicate reliably at scale. If you’ve built communications infrastructure at companies like Vapi, Twilio, Vonage, Telnyx, Bandwidth, LiveKit, or similar organizations—and want to help build the next generation of AI-powered communications—we want to talk to you!!