Voice AI in 2026 is no longer a one-tool decision. ElevenLabs, Vapi, Retell, Cartesia, OpenAI Realtime, and Twilio ConversationRelay each solve a different slice of the problem. Picking wrong costs you 6 weeks of build time and 2× your API bill.
The four jobs of a voice AI stack
Whatever tool you pick, it has to do four things:
- Speech-to-text (STT) — convert what the caller said into text, in real time, with low latency.
- Reasoning (LLM) — decide what to say back, look up info, follow scripts, handle objections.
- Text-to-speech (TTS) — generate a natural-sounding voice response.
- Telephony — connect the whole thing to a real phone number that real people can dial.
Some platforms bundle all four (Vapi, Retell). Some give you one piece (ElevenLabs = TTS only, Cartesia = TTS only, Deepgram = STT only). Knowing what each does — and doesn't — saves you from buying the same capability twice.
The major platforms — head to head
Vapi
What it is: An end-to-end voice agent platform. You bring a system prompt and a Twilio number; Vapi handles the rest.
- Best for: Inbound + outbound voice agents, agencies serving multiple clients, anything that needs to ship in < 1 week.
- Strengths: Fastest to deploy. Excellent function-calling. Native Twilio + Telnyx integration. Real-time transcripts.
- Weaknesses: Per-minute pricing adds up at scale ($0.05–0.13/min depending on voice + model). Dashboard is best-in-class but limits if you want deep customization.
- Cost ballpark: $40–110 per 1,000 minutes, all-in.
Retell
What it is: Vapi's biggest competitor. Same shape (end-to-end voice agents) but with stronger latency and a more developer-first API.
- Best for: Higher-volume call operations, teams with engineers who want fine control.
- Strengths: Sub-500ms response times consistently. Better interruption handling than Vapi. Cleaner SDK.
- Weaknesses: Less polished dashboard. You'll spend more dev time. Smaller community.
- Cost ballpark: $50–120 per 1,000 minutes.
ElevenLabs
What it is: The TTS gold standard. Also offers a full conversational agent now, but the voice quality is the headline.
- Best for: Brand voice cloning. Multilingual deployments. Anywhere voice quality is the differentiator.
- Strengths: Voice quality is unmatched in 2026. Voice cloning from 1 minute of audio. Wide language support.
- Weaknesses: Higher cost than Cartesia for raw TTS. Conversational agent (Convai) is newer than competitors.
- Cost ballpark: Pro tier $99/mo for 100k characters; production deployments scale via API at ~$0.30/1k characters.
Cartesia (Sonic)
What it is: Ultra-low-latency TTS. Their model — Sonic — generates voice in ~90ms.
- Best for: Real-time interactive voice. Anywhere latency feels broken.
- Strengths: Fastest TTS we've tested in 2026 (sub-200ms end-to-end). Quality is competitive with ElevenLabs in English.
- Weaknesses: Smaller voice library. Not as strong outside English.
- Cost ballpark: $5/month base + ~$0.04 per 1k characters.
OpenAI Realtime API + Twilio
What it is: OpenAI's GPT-5 Realtime model speaks natively (no separate TTS), and you connect it to Twilio yourself.
- Best for: Custom builds where you want full control of the loop and don't mind plumbing.
- Strengths: Single-model latency (audio-in, audio-out). Good for natural multi-turn conversations.
- Weaknesses: You're building telephony glue yourself (or via Twilio ConversationRelay). No dashboard. Voice options limited compared to ElevenLabs.
- Cost ballpark: ~$0.06/min audio-in + $0.24/min audio-out at GPT-5 Realtime rates as of Q2 2026.
The decision matrix
If you remember nothing else, remember this:
- Need a working voice agent in < 7 days? → Vapi.
- Need sub-200ms latency for high-stakes calls? → Retell + Cartesia.
- Need a brand-cloned voice for outbound at scale? → ElevenLabs voice → Vapi for telephony.
- Building a custom product, have engineers, want full control? → OpenAI Realtime + Twilio ConversationRelay.
- Need it cheap and don't care about polish? → Twilio Voice Intelligence + open-source Whisper + Coqui TTS. (Save your sanity — pay for the SaaS.)
Don't roll your own STT/TTS pipeline unless you have a real reason. The platforms above are 18 months ahead of anything you'll build internally. The right tradeoff is "buy the platform, customize the prompts, own the data."
What we use for clients
Default stack across our agency deployments in 2026:
- Telephony + orchestration: Vapi
- Voice: ElevenLabs (cloned for high-ticket clients) or Cartesia Sonic (default)
- LLM: Claude 4.5 Sonnet for reasoning, GPT-5 mini for cheap branches
- STT: Deepgram Nova-3 (built into Vapi)
- CRM glue: Go High Level via Vapi's native integration
- Cost per booked appointment: $1.40–$3.20 (vs $40–80 with a human SDR)
When to roll your own
Three legitimate reasons:
- You're running > 50,000 calls/month. The per-minute markup on Vapi/Retell starts to hurt at that scale. Building direct on OpenAI Realtime + Twilio + your own infra can cut costs 40–60%.
- You have unique compliance needs. HIPAA, certain state-level call recording laws, or regulated verticals where you need to own the entire data path.
- You're building a product, not a feature. If voice AI IS your product, you need control of the model, the latency, and the voice — that means custom.
Common mistakes we see
- Picking by price first. A $0.04/min savings is meaningless if booking rate drops 30%.
- Skipping voice quality testing. Always run side-by-side blind tests with real callers. Voice that "sounds fine" in your office sounds robotic on a real phone line.
- Forgetting interruption handling. If callers can't interrupt the AI mid-sentence, the experience is broken. Test it ruthlessly.
- Over-scripting. Long, prescriptive prompts make AI sound robotic. Short, principled prompts let it improvise naturally.
Bottom line
For 95% of service businesses in 2026, the answer is buy. Use Vapi (or Retell), pair with ElevenLabs or Cartesia, and put the engineering effort into the prompts, the script, and the calendar integration — not into reinventing voice infrastructure.
For the other 5% (high-volume, high-stakes, or product-shaped use cases), build on OpenAI Realtime + Twilio. Plan for 6–10 weeks of engineering.