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Once upon a time signaling stack is separated from voice as packet switched SS7 network, with its own protocol stack. SS7 over TCP/IP stack is SIGTRAN. VoIP signaling plane has protocols like H.323 (by ITU), SIP (by IETF) and MEGACO. SIP became most popular. VoIP data plane is RTP. Now in era of Agentic AI, we have  business solutions for different verticals to integrate voice with STT, LLM, TTS etc. Here are few resource URLs

Signalwire

https://www.linkedin.com/posts/briankwest_github-signalwire-demosveronica-this-activity-7430982255675678720-jsTH/

https://developer.signalwire.com/sdks/agents-sdk/

https://github.com/signalwire-demos

https://signalwire.com/

FreeSWITCH

https://en.wikipedia.org/wiki/FreeSWITCH

https://signalwire.com/freeswitch

https://github.com/signalwire/freeswitch

https://developer.signalwire.com/freeswitch/FreeSWITCH-Explained/

https://github.com/amigniter/mod_audio_stream

https://github.com/sptmru/freeswitch_mod_audio_stream

https://medium.com/@srivastava.vikash/day-9-real-time-voice-ai-starts-here-streaming-audio-from-freeswitch-a45d69547164

https://www.cyberpunk.tools/jekyll/update/2025/11/18/add-ai-voice-agent-to-freeswitch.html

Asterisk

https://www.asterisk.org/

https://en.wikipedia.org/wiki/Asterisk_(PBX)

https://github.com/asterisk/asterisk

Plivo

https://www.plivo.com/

https://github.com/plivo

Others

https://medium.com/@dwilkie_34546/implementing-ai-powered-voice-at-somleng-a-technical-deep-dive-93edbb920e02

https://stringee.com/en/

https://www.kamailio.org/w/