Staff Software Engineer (Python, TypeScript/Node.js)
About the Company21X, formerly known as Structurely, has been developing AI-powered communications technology for more than 10 years. The company builds conversational AI solutions that help businesses engage customers, automate conversations, and drive measurable business outcomes. Its unified AI communications platform supports voice, SMS, RCS, email, and other emerging communication channels. The platform serves enterprise customers across mortgage, real estate, home services, and banking, with newer applications in private aviation, sports, and hospitality.21X is a lean, ambitious organization focused on solving complex technical challenges and building communications technology for enterprise-scale use. About the Role21X is looking for a Staff Software Engineer to lead the architecture, development, and ongoing evolution of its AI communications infrastructure. Working alongside executive leadership, you'll have significant technical ownership and direct influence over the platform's architecture and engineering decisions. This is a hands-on engineering position at the intersection of real-time communications, telecommunications infrastructure, modern messaging protocols, and artificial intelligence. You'll build systems that enable AI agents to communicate naturally and reliably with people across voice, SMS, RCS, and other channels. The work spans real-time AI voice infrastructure, SIP and VoIP systems, WebRTC, RCS messaging, audio streaming, carrier integrations, distributed systems, and developer-facing APIs. You'll also work closely with AI engineers to connect communication services with agent orchestration, real-time interactions, and workflow management. The focus is on building and operating production systems rather than managing teams of developers.
Key Responsibilities:
Architect, develop, and optimize the real-time AI voice communications infrastructure.
Build and maintain SIP, VoIP, WebRTC, and telephony systems.
Integrate and optimize carrier connectivity through providers such as Vonage, Twilio, and other platforms.
Develop high-performance, low-latency audio streaming pipelines.
Optimize speech-to-text (STT), text-to-speech (TTS), and AI model integrations.
Troubleshoot 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, call transfers, answering machine detection, and failover capabilities.
Architect and implement RCS Business Messaging (RBM) infrastructure and integrations with Vonage and Google's RCS ecosystem.
Build rich messaging features, including cards, carousels, interactive buttons, suggested replies, and rich media.
Design SMS/MMS fallback mechanisms and reliable message delivery, routing, and event-processing systems.
Implement messaging identity, sender verification, and compliance capabilities.
Develop unified communication services supporting voice, SMS, RCS, and future channels.
Build event-driven workflows, persistent conversation state, and session continuity across communication channels.
Develop communication APIs that connect with the AI agent orchestration platform.
Collaborate with AI engineers on real-time agent interactions, tool execution, and workflow management.
Design scalable, fault-tolerant infrastructure capable of supporting high volumes of AI-powered interactions.
Implement carrier redundancy, intelligent routing, monitoring, logging, tracing, and failover capabilities.
Improve system latency, throughput, uptime, reliability, and infrastructure costs.
Establish engineering standards for performance, testing, security, and production reliability.
Design and develop developer-facing APIs for voice, SMS, RCS, and communication events.
Implement authentication, authorization, tenant isolation, webhooks, and event-streaming infrastructure.
Support usage tracking, metering, billing, SDK development, technical documentation, and developer experience.
Build reusable communications services for internal teams and external developers.
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.
Why Join:
21X?Work at the intersection of AI and telecommunications, bringing together real-time voice, advanced messaging, and AI agents.
Take ownership of core platform architecture and influence important technical decisions.
Solve complex engineering challenges involving the communications infrastructure behind enterprise AI, rather than simply building another LLM wrapper.
Join a 10-year-old startup with capital and cash flow, investing significantly in its future.
Contribute to a growing technology platform serving multiple industries, including mortgage, real estate, insurance, home services, aviation, and sports.
Help develop the communications infrastructure that will enable AI agents to interact with customers across multiple channels.
Location and Work Arrangement:
Detroit, Michigan / Remote.
Full-time employment.
Staff / Principal Engineer seniority.