[feature] Voice AI agents

As a Workflow Developer
I want to be able to build voice-based AI Agents
So that I can create solutions where voice is used for communication

Technical Details
1. Phone-based AI agents
2. Web-based AI Agents
3. Check:
- https://elevenlabs.io/conversational-ai
- https://deepgram.com/solutions/conversational-ai
- https://vapi.ai/
- https://vapi.ai/blog/vapi-vs-twilio-conversation-relay
- https://livekit.io/
- https://www.voiceflow.com/

Acceptance Criteria
TODO

Definition of Done
TODO

No repository files, tests, or entry points are named. Start by reviewing the referenced voice-AI services and the existing workflow and AI-agent architecture, then define acceptance criteria for both phone-based and web-based agents before implementation; completion is currently unspecified because the issue leaves Acceptance Criteria and Definition of Done as TODO.

Written by the indexing model from the issue text.