---
title: "Replace Retell With LiveKit Cloud + OpenAI Realtime"
description: "Retell's flow builder is great for SMB. When you need real WebRTC, video frames and SFU control, LiveKit Cloud + Realtime is the right move. Full code path."
canonical: https://callsphere.ai/blog/vw3h-replace-retell-with-livekit-cloud-realtime
category: "AI Engineering"
tags: ["Retell", "LiveKit", "OpenAI Realtime", "WebRTC", "Tutorial"]
author: "CallSphere Team"
published: 2026-03-25T00:00:00.000Z
updated: 2026-05-07T09:59:34.739Z
---

# Replace Retell With LiveKit Cloud + OpenAI Realtime

> Retell's flow builder is great for SMB. When you need real WebRTC, video frames and SFU control, LiveKit Cloud + Realtime is the right move. Full code path.

> **TL;DR** — Retell ships a polished flow builder and $0.07/min PSTN, but it gates real WebRTC, agent-side video and multi-room orchestration. LiveKit Cloud's official OpenAI Realtime plugin gives you all three on infrastructure OpenAI itself partnered with for Advanced Voice.

## What you'll build

A Python LiveKit agent that connects to a room, talks to OpenAI Realtime, supports video frame input (when the user shares camera), and warm-transfers to a human via SIP. Same UX a Retell flow gives you, but you own the audio path end-to-end.

## Prerequisites

1. Retell account with at least one production flow + 30 days of analytics.
2. LiveKit Cloud project (free tier covers prototyping).
3. OpenAI API key with Realtime access (gpt-4o-realtime-preview).
4. Python 3.11+, `livekit-agents`, `livekit-plugins-openai`.
5. Twilio Elastic SIP for PSTN ingress.

## Architecture

```mermaid
sequenceDiagram
  participant C as Caller (PSTN)
  participant SIP as Twilio Elastic SIP
  participant LK as LiveKit Cloud SFU
  participant AG as Python agent
  participant OAI as OpenAI Realtime
  C->>SIP: PSTN INVITE
  SIP->>LK: SIP -> Room
  AG->>LK: dispatch agent into Room
  AG->>OAI: WSS Realtime
  LKAG: bidirectional Opus
  AG-->>LK: TTS audio frames
  LK-->>SIP: Opus
  SIP-->>C: PSTN audio
```

## Step 1 — Spin up the agent

```python
from livekit import agents
from livekit.agents import AgentSession, Agent, RoomInputOptions
from livekit.plugins import openai

class Receptionist(Agent):
    def **init**(self):
        super().**init**(instructions=open("prompts/recept.md").read())

async def entrypoint(ctx: agents.JobContext):
    session = AgentSession(
        llm=openai.realtime.RealtimeModel(
            voice="alloy",
            model="gpt-4o-realtime-preview-2025-06-03",
        ),
    )
    await session.start(
        agent=Receptionist(),
        room=ctx.room,
        room_input_options=RoomInputOptions(video_enabled=True),
    )

if **name** == "**main**":
    agents.cli.run_app(agents.WorkerOptions(entrypoint_fnc=entrypoint))
```

## Step 2 — Wire Twilio Elastic SIP to LiveKit

LiveKit's SIP service needs an inbound trunk. Create one:

```bash
lk sip inbound create '{
  "name": "twilio-trunk",
  "numbers": ["+18452345678"],
  "auth_username": "lk_user",
  "auth_password": ""
}'
```

Then on the Twilio side point the trunk's "Origination URI" at `sip::5060`.

## Step 3 — Define tools

LiveKit agents register tools via decorators:

```python
from livekit.agents import function_tool, RunContext

class Receptionist(Agent):
    @function_tool
    async def book_appointment(self, ctx: RunContext, name: str,
                               phone: str, slot_iso: str) -> dict:
        return await crm.book(name=name, phone=phone, when=slot_iso)
```

## Step 4 — Warm transfer to a human

```python
@function_tool
async def transfer_to_human(self, ctx: RunContext, reason: str):
    await ctx.proc.job.room.local_participant.publish_data(
        f"transferring: {reason}".encode(), reliable=True)
    await sip.transfer_call(participant=ctx.proc.job.participant,
                            sip_uri="sip:human@your-pbx")
```

## Step 5 — Add video understanding

OpenAI Realtime now accepts video frames. `RoomInputOptions(video_enabled=True)` already enabled it; ask the user to share their camera and the model will consume frames.

## Step 6 — Migrate Retell flows

Retell flow nodes typically map to a single Agent + tools. Where Retell uses "branch" nodes, use `Agent.handoff(another_agent)` for clean specialist transfers.

## Step 7 — Cutover plan

Move 10% of inbound to LiveKit for a week. Compare booking conversion and average handle time. Ramp 10 → 30 → 70 → 100.

## Common pitfalls

- **SIP NAT.** LiveKit Cloud handles SIP traversal; on-prem deployments need a public IP.
- **Plugin version drift.** Pin `livekit-plugins-openai==0.x` — the API moves quarterly.
- **Long-form context.** Realtime drops oldest turns past ~25k tokens; offload to a tool that fetches context on demand.

## How CallSphere does this in production

CallSphere's OneRoof Property stack runs 10 specialists over WebRTC + Pion + NATS — a similar shape to LiveKit but self-hosted for cost at scale. Healthcare uses OpenAI Realtime over FastAPI :8084 with HIPAA logging across 14 tools. Salon runs ElevenLabs voices and `GB-YYYYMMDD-###` references on 4 agents. 37 agents total, 90+ tools, 115+ DB tables, 6 verticals. [/compare/retell](/compare/retell) · [/pricing](/pricing).

## FAQ

**Will Retell's voice match LiveKit's?** Both use OpenAI voices — choose the same `voice` parameter.

**Latency?** LiveKit Cloud + Realtime hits 550–700ms p50.

**SOC2/HIPAA?** LiveKit Cloud is SOC2; HIPAA via BAA.

**Can I A/B inside the same number?** Yes — Twilio's `` weights work fine.

**Outbound campaigns?** Use `livekit-cli sip create-call` to dial out.

## Sources

- [LiveKit OpenAI Realtime plugin](https://docs.livekit.io/agents/integrations/openai/realtime/)
- [LiveKit + OpenAI partnership](https://blog.livekit.io/openai-livekit-partnership-advanced-voice-realtime-api/)
- [Retell pricing](https://www.retellai.com/)
- [/compare/retell](https://callsphere.ai/compare/retell)

---

Source: https://callsphere.ai/blog/vw3h-replace-retell-with-livekit-cloud-realtime