The AI autopilot for your calls.

Zeus is like Claude Code for AI voice agents.

99.4% on Big Bench Audio2¢ a minute and up

Job site, excavator noiseHears callers right — noise, accents and all.

Job site, excavator noise

Flower shop, passcode firstFollows your script — even the tricky parts.

Flower shop, passcode first

Rural Thailand, in ThaiSpeaks 62 languages, switches gracefully.

Rural Thailand, in Thai

Accuracy

The most accurate voice AI. On real calls.

Other voice AI architectures mishear callers and disobey instructions. ThunderPhone’s architecture works better.

Hearing

Several transcribers, plus the audio itself

Names, numbers and mumbles get heard right.

Thinking

A fast model and a thinking model

Switched turn by turn, so rules get followed without lag.

Conversation

Turn detection, noise, overtalk

Callers can interrupt. Background noise can’t.

Speaking

Natural voices, 62 languages

Switches languages mid-call when the caller does.

99.4%on Big Bench Audio: #1 among models fast enough to handle a phone call.Dataset on Hugging Face ↗

Pricing

From 2¢ a minute. Why overpay?

You pay what each call costs us, plus a thin margin. Zeus picks the cheapest setup that does the job, so simple calls stay cheap.

US call centersLive human agent

$0.67+/min

Overseas call centersLive human agent

$0.13+/min

Other voice AIVapi · Retell · Bland

7¢+/min

ThunderPhoneFull-stack voice AI

2¢+/min

US call center bar not to scale. Typical calls at list prices; competitor figures from public pricing.

Cost plus a thin margin

Vendor prices and hosting at cost. Our margin is 17% to 25% of what you pay.

You pick the pricing

Pay whatever it takes to maximize performance, set a cost cap, or optimize for your unit economics.

Price cuts reach you

When a vendor gets cheaper, your calls do too. How pricing works

Zeus

Made possible by Zeus.

Zeus is an AI agent that builds, tests and tunes your voice agent, the way Claude Code ships software. Tell it what your calls should do.

01
Build
From recordings, a script, a conversation, or a one-click import from Vapi, Retell, ElevenLabs or Bland.
02
Connect
Phone systems and integrations, set up for you.
03
Test
Simulated callers and replays of your past calls.
04
Improve
Spots problems on live calls and proposes A/B tests.

ZeusNorthside Insurance · front desk

Callers keep hanging up when we ask for their policy number.

ZeusMost hang-ups follow that question. I tried looking the policy up from their phone number instead. In tests, it kept every booking that worked before.

60 of 60 simulated and replayed calls pass. 2 bookings recovered.

The problem

Why other voice AI fumbles.

Most other voice AI platforms use one of two architectures. Both make mistakes that business calls can’t afford.

Three-step pipeline

One guess at what the caller said

The transcriber boils rich audio down to one best guess. When it’s wrong, the LLM can’t recover.
Fast LLMs make mistakes. Careful ones take seconds to answer.
Rigid turn-taking makes it talk over people or wait too long.

Speech-to-speech model

Natural, but unpredictable

Sounds great in a demo.
Unreliable at following instructions and calling tools.
Erratic behavior is a real risk on business calls.

ThunderPhone

Many models per call, tuned to yours

Hears what was said, even when one transcriber misses.
Answers fast, and thinks harder when a turn needs it.
Feels more like talking to a person.

On real calls

What that looks like on a call.

ThunderPhone gets these right where other voice AI gets them wrong.

Hears what callers actually say.

Several transcripts plus the raw audio, so addresses, spellings and mumbles don’t turn into wrong answers.

AgentDo you own or rent?

CallerI rent.

Typical voice AI

Heard“Hi Ron”

AgentIt sounds like you’re talking to someone in the background. I was asking if you’re a homeowner?

ThunderPhone

Heard“I rent” “I rent” “Hi Ron” + audio

AgentSo you rent. And what’s your address?

Follows your script, even the tricky parts.

A thinking model steps in on turns where a rule matters. Everything else stays snappy.

CallerHow much ibuprofen can I take?

Typical voice AI

AgentTake 800 mg every 8 hours.

Replies instantly, ignoring your rule to never give dosage advice.

ThunderPhone

AgentLet me check on that for you. … Sorry, I can’t give dosage advice.

Checks your rule before it answers.

Talks like a person, not a phone tree.

Backchannels like “mm-hm” don’t stop it. Interruptions do, instantly.

AgentWe’re open Monday to—

CallerJust the weekend hours?

Typical voice AI

Agent—Friday nine to five. Is there anything else I can help you with today?

ThunderPhone

AgentSaturday ten to two.

Latency

About 1.5 seconds on a typical turn.

Most turns can be answered fast. When one needs real thought, ThunderPhone takes a bit longer to answer correctly.

Every call needs something different. Zeus decides what.

Every turn of every call does better with a different set of models behind the scenes. Zeus navigates those tradeoffs.

Part of the call	Dental front desk	Outbound sales
Hearing	Three transcribers plus the audio, for names and dates of birth	One fast transcriber plus the audio
Thinking	Thinking model on insurance and medical questions	Fast model on most turns, thinking model on objections
Turn-taking	Patient: waits out pauses from older callers	Snappy: answers fast, yields to interruptions
Voice	Platform voice, steady on numbers	Premium voice, most natural

Example picks. Zeus makes them from your calls and your goals.

Why Zeus

Why Zeus works where coding agents don’t.

We built Zeus after years of working alongside Claude Code and Codex. They’re great at most software, but less good at voice agents.

Why coding agents struggle

Trained on text, not calls. What little audio they saw came from places like YouTube, not real call centers.
The know-how isn’t online. What is online is full of exaggerated marketing claims, and AI tends to believe them blindly.
No closed loop. They can’t run a call or spot what went wrong in one, so they thrash.

What Zeus has

Evals on real calls. We test every model on real call data, not vendor claims, and know which is faster and which is smarter.
Two years of know-how. We’ve built what we’ve learned building voice agents into Zeus’s prompts, guardrails and tools.
Call tools. It simulates hard calls and spots issues on live ones, so it confirms a change works before it calls it done.
Your calls. It learns from how your conversations go and backtests every change against them.

Zeus, start to finish

Live on real calls with far less effort.

Zeus handles the annoying parts: SIP trunks, VoIP settings and integrations.

1Start from what you have

Import from Vapi, Retell, ElevenLabs or Bland. Zeus handles it end to end.

2Connect

Numbers, carriers and the tools your calls touch, set up and checked.

3Test

Simulated callers, text and audio, plus replays of your past calls.

4Improve

Finds problems on live calls and tests fixes on a slice of traffic, measured against your funnel goals.

You decide how much autonomy Zeus gets. Every change ships as a new version.

How to use it

Use it your way.

On the phone, on your website or in your app. Inbound and outbound.

Web app

The full Zeus experience

Build, test and watch your calls in one place.

Your coding agent

Claude Code, Codex and MCP

Plugins give your own agent ThunderPhone’s tools, many of them the ones Zeus uses.

Frameworks

LiveKit Agents and Pipecat

Run ThunderPhone as the speech-to-speech model in your stack.

API

Custom integrations

POST /v1/calls { "agent": "front-desk", "to": "+1 415 555 0100" }

Outbound

Campaigns built in

Upload a CSV of up to 5,000 contacts, set calling windows by time zone and retries, and follow progress live.

White label

Under your brand

Client portals and custom work for agencies and platforms. Enterprise

Read the docs for the API, MCP server and plugins.

Built for production

Ready for the real world.

The details that make or break real calls, and the requirements of healthcare, the EU and more.

Transfers

Warm, cold and whisper

Hand off to a person with context.

Outbound

Voicemail and screeners

Detected on every outbound call.

Oversight

Live supervision

Listen in and whisper guidance mid-call.

Compliance

HIPAA and GDPR

BAA and DPA at no extra charge.

Languages

Understands accents and mixed-language calls, and switches languages mid-call.

EnglishEspañolFrançaisDeutschPortuguêsहिन्दी日本語中文العربية한국어ItalianoFilipino

All 62 languages

Enterprise

1¢ a minute or less, at volume.

Fine-tuned models for high-volume calls, custom integrations, white label, data residency and your compliance needs.

Talk to us Enterprise

Our story

We built the voice platform we needed.

Founded by Stanford AI Lab and Y Combinator alumni working on voice AI since 2024. We started on car-insurance pre-sales, where off-the-shelf platforms misheard callers, broke rules and cost too much.

So we built our own stack, then tuned it by hand for enterprise customers. Zeus is the next step: the AI that does that tuning for every customer, totally hands-off.

Our job is to make ThunderPhone handle your calls at the highest performance today’s AI allows, at the lowest prices possible. If we ever fall short, email me personally.

Alex KolchinskiCEO, ThunderPhonealex@thunderphone.com

Tell Zeus what your calls should do.

Zeus is free for now. Calls from 2¢ a minute.

Need something custom? Talk to us