Software 4tgG9Azg - Remote Work — United States

About the Role

21X, formerly known as Structurely, is seeking a Staff Software Engineer to lead the architecture, development, and evolution of its AI communications infrastructure. This is a highly technical, hands-on role with significant ownership and architectural influence, working alongside executive leadership.

You will work at the intersection of real-time communications, telecommunications infrastructure, modern messaging protocols, and artificial intelligence. The focus is on building systems that enable AI agents to communicate naturally, reliably, and intelligently across voice, SMS, RCS, email, and other channels. You will help solve complex engineering challenges involving real-time voice quality, messaging experiences, AI integrations, and enterprise-scale infrastructure.

Key Responsibilities

Real-Time AI Voice Infrastructure

• Architect, build, and optimize the real-time AI voice communications stack.

• Develop and maintain SIP, VoIP, WebRTC, and telephony infrastructure.

• Integrate and optimize carrier connectivity through 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.

• Address audio engineering challenges involving codecs, transcoding, jitter, packet loss, and network latency.

• Improve voice quality and conversational responsiveness over traditional telephone networks.

• Develop call routing, transfers, answering machine detection, and failover capabilities.

• Build systems designed for high concurrency, reliability, and enterprise-scale traffic.

RCS and Next-Generation Messaging Infrastructure

• Lead the technical development of 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.

• Improve messaging performance, deliverability, and cost efficiency.

• Develop interactive RCS experiences within native messaging environments.

Cross-Channel AI Communications

• Build the communications layer that enables AI agents to interact with customers across multiple 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 that support persistent conversation state and session continuity.

• Build communication APIs that integrate with the AI agent orchestration platform.

• Collaborate with AI engineers on real-time agent interactions, tool execution, and workflow management.

• Maintain consistency, reliability, and observability across communication channels.

Enterprise Infrastructure, Reliability, and Scale

• Design scalable, fault-tolerant communications systems capable of supporting millions of AI-powered interactions.

• Build high-concurrency voice and messaging infrastructure.

• Implement carrier redundancy, intelligent routing, and failover capabilities.

• 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.

Developer Platform and Communications APIs

• 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 for internal teams and external developers.

• Collaborate on SDKs, technical documentation, and developer experience.

Required Qualifications

• 8+ years of professional software engineering experience, including 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 actively involved in writing production code.

Preferred Qualifications

• 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, or 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.

About 21X

21X, formerly Structurely, has been developing AI-powered communications technology for more than 10 years. Its unified AI communications platform supports voice, SMS, RCS, email, and other emerging communication channels.

The company serves enterprise customers across mortgage, real estate, home services, and banking, and has expanded into private aviation, sports, and hospitality. Its focus is on helping businesses engage customers, automate conversations, and achieve measurable business outcomes.

Why Join 21X

• Take significant architectural ownership and directly influence the company's core platform.

• Work on real-time voice, advanced messaging, and AI agent infrastructure.

• Solve complex engineering challenges beyond building applications around large language models.

• Join a well-funded, cash-flow-generating startup investing in its future.

• Contribute to a growing technology platform serving multiple industries.

• Help shape the next generation of AI-powered business communications.

Position Details

• Job Title: Staff Software Engineer — AI Communications Platform.

• Seniority: Staff / Principal Engineer.

• Employment Type: Full-Time.

• Location: Detroit, MI / Remote.