Retell AI vs Telnyx: Which Voice AI Platform Should You Build With?

I've spent the last year building conversational AI systems, and two platforms keep coming up: Retell AI and Telnyx. They both let you build voice agents, but they're solving different problems.

Retell is a purpose-built conversational AI platform. Telnyx is a telecom infrastructure provider that added AI capabilities. That distinction matters more than you'd think.

Here's what I've learned from working with both.

The Core Difference

Retell AI is laser-focused on conversational experiences. You get built-in LLM orchestration, memory management, and turn-taking logic out of the box. It's opinionated — which is exactly what you want when you're trying to ship fast.

Telnyx gives you the primitives: SIP trunking, WebRTC, SMS, and AI features bolted on top. You build the orchestration yourself. More control, more work.

If you're building a phone tree with some AI sprinkled in, Telnyx makes sense. If you're building an actual conversational agent, Retell saves you weeks.

Voice Quality & Latency

Retell uses WebRTC under the hood and supports multiple TTS providers (ElevenLabs, PlayHT, OpenAI). I consistently see 600-800ms latency from user speech to agent response. That's fast enough to feel natural.

Telnyx's latency depends on how you architect it. If you're using their AI services directly, you're looking at 800ms-1.2s. Not terrible, but noticeable. If you bring your own TTS/STT and optimize the pipeline, you can get it closer to Retell's numbers — but now you're maintaining that infrastructure.

Both platforms handle call quality well. No complaints on either side.

LLM Integration

Retell: First-class support for OpenAI, Anthropic, and custom models. You define your agent's personality, tools, and conversation flow in their dashboard. It handles function calling, context management, and prompt engineering patterns automatically.

The workflow looks like this:

// Define tools your agent can use
const tools = [
  {
    name: 'check_availability',
    description: 'Check if a time slot is available',
    parameters: { /* your schema */ }
  }
];

// Retell handles the orchestration

Telnyx: You manage the LLM integration yourself. They provide webhooks and streaming APIs, but the prompt engineering, tool orchestration, and state management? That's on you.

For simple use cases, that's fine. For complex multi-turn conversations with function calling and memory, you're rebuilding what Retell gives you.

Developer Experience

Retell's API is clean. You create an agent, configure it via dashboard or API, and get a phone number or embed it in your app. Their docs are solid, though sometimes light on advanced use cases.

Telnyx's API is massive. You're working with telecom primitives, so there's more surface area. The docs are comprehensive but dense. Expect a steeper learning curve.

Retell has built-in debugging tools — you can replay conversations, see the full transcript with timestamps, and inspect every LLM call. This is huge when something breaks in production.

Telnyx gives you logs and webhooks. You build the debugging tools yourself.

Pricing Models

This is where it gets interesting.

Retell: Pay per minute of conversation ($0.10-0.15/min depending on volume), plus your LLM and TTS costs. It's predictable. No hidden fees.

Telnyx: Pay for the telecom layer (inbound/outbound minutes, phone numbers) plus AI service usage separately. If you're using their AI features, costs can stack up. But if you bring your own models and just use their SIP infrastructure, you can optimize heavily.

For a typical customer service agent handling 10,000 minutes/month:

• Retell: ~$1,000-1,500 + LLM costs
• Telnyx: ~$400-600 (telecom) + whatever you spend on AI services

Retell is more expensive, but you're paying for less engineering time.

Unique Features

Retell:

• Built-in interruption handling (users can cut off the agent naturally)
• Conversation memory across calls
• Pre-built templates for common use cases
• Multi-language support with automatic detection

Telnyx:

• Global SIP infrastructure (if you need international reach)
• SMS/MMS alongside voice
• Number porting and enterprise telecom features
• More flexibility with custom audio processing

Where Each Shines

Use Retell if:

• You're building conversational AI as the core product
• You want to ship fast and iterate
• You need interruption handling and natural turn-taking
• Developer time is more expensive than platform costs

Use Telnyx if:

• You're adding AI to existing telecom infrastructure
• You need SMS, MMS, and voice in one platform
• You have engineers who want full control
• You're optimizing for cost over development speed

My Take

I reach for Retell when I'm building conversational experiences. The time savings are massive, and the conversation quality is better out of the box.

I use Telnyx when I need telecom primitives or when the client already has Telnyx infrastructure and wants to add AI features.

Both are solid platforms. The right choice depends on whether you're building conversation-first (Retell) or infrastructure-first (Telnyx).

If you're just getting started with voice AI, go with Retell. You'll ship faster and learn what actually matters before you start optimizing costs.