IMS Engineer

Telnyx is a financially stable and profitable industry leader building the future of global connectivity through a private, global, multi-cloud IP network and hyperlocal edge technology. They are productizing a carrier-grade IMS core on Kamailio and seeking an IMS Engineer with VoLTE expertise and the ability to modify Kamailio's C codebase. The role involves designing, implementing, and operating P-/I-/S-CSCF functions at a global scale, focusing on call flows, module configuration, HSS/UDM/UDR integration, productization, performance tuning, reliability, and security.

Must Have

• VoLTE expertise: Hands-on IR.92 (voice/SMS), registration/attach flows, SIP/SDP (AMR-WB/EVS), QoS/bearers via PCRF/PCF, emergency calling, multi-vendor interop.
• Core IMS/SIP: 5+ years with SIP/IMS; strong grasp of 3GPP (TS 23.228, 24.229, 29.xx), CSCF roles, registration, service routing.
• Kamailio mastery: Confident authoring kamailio.cfg routing logic; registrar/dialog/topology hiding; IMS modules in production.
• Software engineering to modify Kamailio: Proficient in C for high-performance network services; experience building and changing Kamailio core/modules.
• Tooling: gcc/clang, make, gdb, valgrind/AddressSanitizer, perf/flamegraph, packet tools (pcap, sngrep, ngrep).
• Strong unit/functional testing habits; familiarity with Kamailio test approaches and CI.
• Comfortable proposing and landing upstream PRs, engaging in code review, and maintaining forks/patch sets.
• Systems & SRE: Linux networking, DNS (NAPTR/SRV), TLS/mTLS, observability (Prometheus/Grafana, ELK/Loki/OpenTelemetry).
• Automation: Git workflows, CI/CD, Docker/Kubernetes, IaC (Terraform/Helm), scripting (Python/Bash/Lua).
• Communication: Clear design docs and incident write-ups.

Good to Have

• FreeSWITCH/Asterisk; RTP/RTCP; media relays (rtpengine/rtpproxy); SBC concepts.
• Diameter (Cx/Sh/Gx), ENUM/NP; VoWiFi/VoNR; SRVCC/eSRVCC exposure.
• Prior upstream contributions to Kamailio or related SIP projects.
• Telecom fraud mitigation and signaling security patterns.

About Telnyx

Telnyx is an industry leader that's not just imagining the future of global connectivity—we're building it. From architecting and amplifying the reach of a private, global, multi-cloud IP network, to bringing hyperlocal edge technology right to your fingertips through intuitive APIs, we're shaping a new era of seamless interconnection between people, devices, and applications.

We're driven by a desire to transform and modernize what's antiquated, automate the manual, and solve real-world problems through innovative connectivity solutions. As a testament to our success, we're proud to stand as a financially stable and profitable company. Our robust profitability allows us not only to invest in pioneering technologies but also to foster an environment of continuous learning and growth for our team.

Our collective vision is a world where borderless connectivity fuels limitless innovation. By joining us, you can be part of laying the foundations for this interconnected future. We're currently seeking passionate individuals who are excited about the opportunity to contribute to an industry-shaping company while growing their own skills and careers.

Telnyx is productizing a carrier-grade IMS core on Kamailio. You’ll design, implement, and operate P-/I-/S-CSCF functions at a global scale—with VoLTE expertise and the ability to modify Kamailio’s C codebase when needed.

What you’ll do

• Design & build: Own P-/I-/S-CSCF call flows; configure Kamailio modules (ims\_*, dispatcher, topos, tm, usrloc, permissions); integrate HSS/UDM/UDR.
• Productize: Turn prototypes into supportable services with versioned configs, CI/CD, and observability.
• Performance & scale: Tune throughput, dialog/state handling, NAT traversal, DNS/SRV, TLS/mTLS; benchmark CPS/latency.
• Reliability: Architect HA/DR; lead load tests, game days, incident reviews.
• Security: Implement abuse/DoS controls (pike/htable/rate limiters), auth, encryption, and audit-grade logging.
• Upstream: Contribute fixes/patches and collaborate with the Kamailio community.
• On-call (shared): Keep the voice core healthy.

What you’ll bring (must-haves)

• VoLTE expertise: Hands-on IR.92 (voice/SMS), registration/attach flows, SIP/SDP (AMR-WB/EVS), QoS/bearers via PCRF/PCF, emergency calling, multi-vendor interop.
• Core IMS/SIP: 5+ years with SIP/IMS; strong grasp of 3GPP (TS 23.228, 24.229, 29.xx), CSCF roles, registration, service routing.
• Kamailio mastery: Confident authoring kamailio.cfg routing logic; registrar/dialog/topology hiding; IMS modules in production.
• Software engineering to modify Kamailio (required):
• Proficient in C for high-performance network services (memory management, concurrency, lock-free/atomic patterns).
• Experience building and changing Kamailio core/modules (parsers, transaction/dialog layers, ims\_*), adding/adjusting AVPs/headers, and writing new module logic.
• Tooling: gcc/clang, make, gdb, valgrind/AddressSanitizer, perf/flamegraph, packet tools (pcap, sngrep, ngrep).
• Strong unit/functional testing habits; familiarity with Kamailio test approaches and CI.
• Comfortable proposing and landing upstream PRs, engaging in code review, and maintaining forks/patch sets.
• Systems & SRE: Linux networking, DNS (NAPTR/SRV), TLS/mTLS, observability (Prometheus/Grafana, ELK/Loki/OpenTelemetry).
• Automation: Git workflows, CI/CD, Docker/Kubernetes, IaC (Terraform/Helm), scripting (Python/Bash/Lua).
• Communication: Clear design docs and incident write-ups.

Nice to have

• FreeSWITCH/Asterisk; RTP/RTCP; media relays (rtpengine/rtpproxy); SBC concepts.
• Diameter (Cx/Sh/Gx), ENUM/NP; VoWiFi/VoNR; SRVCC/eSRVCC exposure.
• Prior upstream contributions to Kamailio or related SIP projects.
• Telecom fraud mitigation and signaling security patterns.