Telnyx is hiring an IMS Engineer

About the Role

Role details below.

Responsibilities

• 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

Requirements

• 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

Additional Information

• On-call (shared): Keep the voice core healthy