Software Engineer C#

Guardian is a call taking & dispatching platform used by emergency service operators across the UK & Europe. It helps 999 or 112 operators answer calls from the public and dispatch nearby responders, enabling the right response when every second counts. The Guardian team is distributed across the UK & Poland, with offices in Kraków and Glasgow, and currently has around 150 employees.

• 2+ years of professional experience with asynchronous network programming in C# / .NET Framework / .NET Core.
• Experience with WCF, SignalR, EF, ASP.NET, WebAPI.
• Understanding of the SIP Protocol Stack (INVITE, PUBLISH, SUBSCRIBE, etc.) and experience with RTP/RTCP stacks.
• Proficiency in parsing complex XML schemas (XSD) and handling multipart/mixed MIME bodies.
• Strong grasp of OOP, SOLID principles, and modern CI/CD workflows (Git, Pull Requests, Agile).

Nice to have:

• Familiarity with TS 24.379 (MCPTT) or related MCX standards.
• Experience with SRTP, TLS, or Key Management lifecycles (KMS).

• Develop C# components for high-concurrency SIP sessions, managing complex headers and MIME bodies to support features like "First-to-Answer" and Group Regroups.
• Implement RTP/RTCP media handling for Mission Critical Voice (MCPTT) and Video (MCVideo), ensuring seamless floor control in half-duplex environments.
• Build handlers for MCData services (SDS, File Distribution) using HTTP and SIP MESSAGE requests, and manage XCAP retrieval logic for XML configuration documents.
• Integrate OIDC authentication flows and MIKEY-SAKKE encryption to ensure media transport remains secure and resilient.