How to Add a Real Phone Number to Your Vapi, Retell, or ElevenLabs Voice AI Agent [2026]

Voice AI platforms like Vapi, Retell, and ElevenLabs Conversational AI have solved the hard part: streaming a low-latency, natural-sounding AI voice. But none of them give you the thing every real-world voice agent needs — a real phone number people can actually call.

Most developers end up stitching together Twilio SIP trunks, wrestling with TwiML, and paying for tools that weren't designed for autonomous agents. This guide shows you the shortcut: use AgentCall as the phone layer and let your voice AI platform do what it does best.

Why Voice AI Platforms Need a Separate Phone Layer

Vapi, Retell, and ElevenLabs focus on conversational AI: speech-to-text, language models, voice synthesis, turn-taking, interruptions. They all accept audio from a caller and stream audio back. What they don't do natively:

• Provision real SIM-backed phone numbers that banks and services trust
• Receive inbound SMS (for OTP, customer replies, 2FA)
• Give every AI agent in your app its own isolated number
• Handle carrier-level number portability, compliance, and DNC lists

That's where AgentCall slots in. You get the phone number, the inbound webhook, the SMS inbox — and you point the voice leg at whichever voice AI platform you prefer.

The Generic Pattern (Works for Any Voice AI Platform)

All three platforms follow the same shape:

1. Provision a phone number on AgentCall
2. Configure a webhook URL pointing at your backend
3. When a call comes in, your backend hands the audio leg to Vapi / Retell / ElevenLabs
4. The voice AI talks to the caller; your backend logs the transcript

Step 1 is identical for all three. Sign up at agentcall.co/sign-up and provision a number with one API call:

curl -X POST https://api.agentcall.co/v1/numbers/provision \
  -H "Authorization: Bearer ac_live_your_key" \
  -H "Content-Type: application/json" \
  -d '{"country": "US", "type": "local"}'

Response:

{
  "id": "num_abc123",
  "number": "+13145550123",
  "type": "local",
  "status": "active",
  "monthlyRate": 0
}

On the Free tier your first number is $0/month. Pro users ($19.99/mo) get unlimited numbers at $2/mo each.

Option 1: Add a Phone Number to Vapi

Vapi natively supports bringing your own SIP trunk. You can point Vapi at the AgentCall number by using AgentCall's webhook as the inbound bridge.

Easiest path: have AgentCall forward inbound calls to your backend, then call Vapi's POST /call endpoint to start a Vapi-managed voice session tied to the incoming caller.

// Your backend endpoint that AgentCall calls on inbound
app.post("/voice/inbound", async (req, res) => {
  const { from, to, callId } = req.body;

  const vapiCall = await fetch("https://api.vapi.ai/call", {
    method: "POST",
    headers: {
      Authorization: `Bearer ${process.env.VAPI_API_KEY}`,
      "Content-Type": "application/json",
    },
    body: JSON.stringify({
      assistantId: process.env.VAPI_ASSISTANT_ID,
      customer: { number: from },
      phoneNumberId: process.env.VAPI_PHONE_NUMBER_ID,
    }),
  });

  res.json({ status: "forwarded" });
});

Option 2: Add a Phone Number to Retell AI

Retell exposes a similar create-phone-call endpoint. The same pattern works: AgentCall receives the inbound call, your webhook initiates a Retell session.

app.post("/voice/inbound", async (req, res) => {
  const { from, to } = req.body;

  await fetch("https://api.retellai.com/v2/create-phone-call", {
    method: "POST",
    headers: {
      Authorization: `Bearer ${process.env.RETELL_API_KEY}`,
      "Content-Type": "application/json",
    },
    body: JSON.stringify({
      from_number: to,
      to_number: from,
      agent_id: process.env.RETELL_AGENT_ID,
    }),
  });

  res.json({ status: "forwarded" });
});

Option 3: Add a Phone Number to ElevenLabs Conversational AI

ElevenLabs Conversational AI (their 2024 voice-agent product) accepts inbound calls through a similar webhook bridge pattern. Provision the AgentCall number, set the inbound webhook to your ElevenLabs-facing handler:

app.post("/voice/inbound", async (req, res) => {
  const { from } = req.body;

  await fetch(
    `https://api.elevenlabs.io/v1/convai/agents/${process.env.ELEVENLABS_AGENT_ID}/phone`,
    {
      method: "POST",
      headers: {
        "xi-api-key": process.env.ELEVENLABS_API_KEY,
        "Content-Type": "application/json",
      },
      body: JSON.stringify({ phone_number: from }),
    },
  );

  res.json({ status: "forwarded" });
});

Check the latest ElevenLabs Conversational AI docs for the exact phone payload shape — they iterate quickly.

When to Skip the Voice AI Platform Entirely

AgentCall also ships its own AI voice calling via initiate_ai_call. You pass a system prompt, a first message, and a target phone number; AgentCall runs the full call and returns a transcript. No Vapi / Retell / ElevenLabs required.

curl -X POST https://api.agentcall.co/v1/calls \
  -H "Authorization: Bearer ac_live_your_key" \
  -H "Content-Type: application/json" \
  -d '{
    "type": "ai",
    "to": "+12125550100",
    "from": "+13145550123",
    "aiSystemPrompt": "You are a friendly appointment scheduler...",
    "aiFirstMessage": "Hi, this is Alex calling from Acme Dental."
  }'

This is the simpler choice for straightforward use cases: appointment reminders, lead qualification, outbound survey calls. If your agent needs deep customization of voice characteristics, interruption handling, or function calling mid-call, pair AgentCall with a dedicated voice AI platform instead.

Why Not Just Use Twilio?

Twilio works, but it was built for 2015-era contact centers, not autonomous agents in 2026. Specifically:

• Twilio VoIP numbers get flagged and blocked by OTP senders like Stripe, Google, and banks — so your agent can't sign up for anything
• Pricing is opaque: you pay for the number, the minutes, the TwiML rendering, the SIP trunk, the carrier surcharges
• No concept of a “per-agent” inbox — provisioning isolated numbers for each of your AI agents requires building a whole abstraction layer yourself
• No OTP extraction, no SIM numbers, no agent-native webhooks

We wrote more about this in Why Twilio and VoIP Don't Work for AI Agents.

Pricing Recap

• Free: 1 phone number at $0/month, 10 SMS/month, 5 voice minutes — no credit card charged
• Pro: $19.99/month base + $2/mo per extra number, $0.015/SMS, $0.035/min voice, $0.20/min AI voice
• First SIM number is included on Pro (SIMs don't get flagged by OTP senders the way VoIP does)

Next Steps

• Sign up for free and provision your first phone number
• Voice AI Infrastructure: Phone Numbers for AI Voice Agents
• 5 Best Phone APIs for AI Agents in 2026
• How to Give Every AI Agent Its Own Phone Number
• Why Twilio and VoIP Don't Work for AI Agents
• Full API documentation

Questions? Email us at support@agentcall.co.