SIP Intercom Station Software

Client needs a VoIP Intercom Station operating in Standards Based SIP.  Developer must have experience working with SOC like NanoPI and OrangePI.  Developer must meet the following list of requirements so as not to waste our time.
Embedded Linux Development
Strong experience developing software for embedded Linux systems
Experience with custom Linux distributions (Debian/Ubuntu)
Kernel configuration, device drivers, and hardware interface support
Bootloader and system startup (U-Boot or similar)
Experience working with flash-based storage systems
Linux system optimization for embedded hardware
VoIP / SIP / Networking
Experience implementing or maintaining SIP-based communications systems
Knowledge of RTP / RTCP media streaming
Familiarity with VoIP codecs such as G.711, G.729, Speex 
Understanding of NAT traversal, STUN, QoS (DSCP), and SIP registration
Experience integrating with PBX systems (Asterisk, FreeSWITCH, etc.)
TCP/IP networking, DHCP, DNS, NTP
Audio Processing
Experience working with real-time audio streaming
Knowledge of ALSA or similar Linux audio frameworks
Audio mixing, buffering, and jitter control
Experience with microphone, speaker, and audio codec hardware
Signal processing basics (tone generation, filtering)
Embedded Hardware Integration
Experience interfacing with GPIO, audio codecs, microphones, and speakers
Knowledge of Ethernet and PoE hardware integration
Experience integrating external peripherals such as:
Footswitch
Handset/headset interfaces
Experience working with low-level hardware debugging
Web Interface Development
Experience building embedded web configuration interfaces
Knowledge of:
HTTP/HTTPS servers on embedded devices
HTML / CSS / JavaScript UI
Backend configuration interfaces (CGI, Python, C, or similar)
Firmware Development & Deployment
Experience building firmware update mechanisms
Secure update strategies and version management
Experience packaging firmware images for embedded devices
Ability to maintain backward compatibility where possible
System Diagnostics & Logging
Experience implementing Syslog, logging systems, and diagnostic tools
Network packet capture and debugging tools
Troubleshooting SIP and RTP traffic
Security
Experience implementing TLS / SRTP secure communications
Secure authentication and credential handling
Secure firmware update processes
Development Tools & Practices
Strong experience with:
C / C++ (primary for embedded Linux)
Bash scripting
Version control (Git)
Cross-compilation toolchains
Debugging tools (GDB, Wireshark, tcpdump)
Nice-to-Have (Bonus Skills)
Experience with radio communication systems or dispatch consoles
Familiarity with DTMF signaling and tone control
Experience developing real-time systems
Experience with multicast audio streaming
 Prefer Developers in US & Canada, but will consider others that demonstrate the above capabilities.
 
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