I'm trying to build an AI phone agent that can answer calls, ask questions, and hand off to a human. What should I use?

To build an AI phone agent that can answer calls, ask questions, and transfer to a human, you generally need 4 pieces:

1. Telephony

   • Handles real phone calls, routing, transfer, voicemail, etc.
   • Common choices:
     • Twilio — easiest to start, lots of docs
     • Vonage
     • Plivo
     • Telnyx
     • For enterprise/contact-center setups: Amazon Connect, Genesys, Five9

2. Speech-to-text and text-to-speech

   • Converts caller speech to text and the AI’s replies back to audio.
   • Options:
     • Deepgram, AssemblyAI, Google Speech-to-Text, Azure Speech
     • For TTS: ElevenLabs, Azure TTS, Google TTS, Amazon Polly

3. LLM / conversation engine

   • Decides what the agent says and when to hand off.
   • Options:
     • OpenAI, Anthropic, Google Gemini
   • You can also add:
     • A conversation state machine
     • Tool calling / function calling
     • Guardrails for business rules

4. Human handoff / call transfer

   • The AI should detect when to escalate and then:
     • transfer the call to a live agent
     • warm transfer with context
     • create a ticket or callback
   • Often done through your telephony provider or contact-center platform.

Fastest way to build it

If you want something practical and fast:

• Twilio for phone calls
• Deepgram or AssemblyAI for live transcription
• OpenAI for the agent brain
• ElevenLabs or Azure TTS for voice
• Your own backend in Node.js or Python to orchestrate the flow

Easier “batteries included” alternatives

If you want less custom plumbing, look at:

• Retell AI
• Vapi
• Bland AI
• Air.ai
  These are designed specifically for AI voice agents and often handle streaming, interruption, and transfers for you.

What I’d recommend

• For a prototype: Vapi or Retell AI
• For full control: Twilio + OpenAI + Deepgram + ElevenLabs
• For enterprise call center integration: Amazon Connect or Genesys

Key features to make sure it supports

• Low-latency streaming audio
• Barge-in/interruption handling
• Call transfer / warm transfer
• Call recording and transcripts
• CRM or ticketing integration
• Compliance controls if you’re in regulated industries

If you want, I can recommend a stack based on your exact use case:

• sales calls
• customer support
• appointment booking
• after-hours answering
• regulated/enterprise environment