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Retell AI vs Ultravox.ai

Two fact sheets from our own research, side by side — pricing, platforms, API access and status re-checked by us rather than quoted from the vendors. Retell AI starts at $0.07, Ultravox.ai at $0.05. Both have a free tier. Both ship a public API.

Retell AIBuild and run AI phone agents that book, qualify and transfer calls, billed by the minute.Ultravox.aiSpeech-to-speech model and API for voice agents that skips the transcription step.

Side by side

FactRetell AIUltravox.ai
CategoryVoice Agents & Phone AIVoice Agents & Phone AI
Pricing modelFreemiumFreemium
Starts at$0.07 minute$0.05 minute
Public APIYesYes
PlatformsWeb, APIWeb, API
CompanyRetell AI Inc.Fixie.ai Inc.
Launched2023—
Last verifiedSep 6, 2026Sep 6, 2026

What our research says

Retell AI

One of the more honest voice-agent stacks: the pricing page breaks out infrastructure, TTS and LLM cost per minute so you can predict a call's cost instead of guessing. Telephony choice is wide and the concurrency allowance is generous. The catch is that per-minute billing plus add-ons makes a busy contact centre expensive fast, and the cheap end of the range only holds if you pick the cheap models.

Pros

  • Per-minute cost broken down by voice infrastructure, TTS provider and LLM on the public pricing page
  • Telephony via SIP trunking, Twilio and Vonage plus n8n, HubSpot, Make and Go High Level integrations
  • 20 concurrent calls included before any concurrency charge
  • Voice, chat and SMS agents share one platform and API
  • HIPAA path exists via a signed BAA covered in the terms of service

Cons

  • Cost per minute more than quadruples between the cheapest and most capable model combinations
  • Knowledge base, denoising, PII removal and branded caller ID are all metered extras
  • No flat-rate plan, so a spike in call volume lands straight on the invoice

Ultravox.ai

One of the few voice stacks where the model itself is the product rather than glue over someone else's transcriber, and the open weights on Hugging Face mean you are not locked into the hosted service. Skipping the speech-to-text hop is the whole point — latency and prosody both benefit. It is a builder's tool, not a no-code agent designer, so budget engineering time as well as per-minute cost.

Pros

  • Model takes audio directly, removing the transcription hop from the response path
  • Open-weight models published on Hugging Face allow self-hosting
  • REST API plus web and mobile SDKs, and SIP for phone calls
  • RAG corpora and custom voices are part of the platform, not add-on services
  • Pay as you go has no monthly floor, so a prototype costs only what it talks

Cons

  • Free tier is limited to 5 concurrent calls, so load testing needs a paid plan
  • Custom voices and RAG corpora are capped even on the $100 Pro tier
  • Threads bill separately per token on top of per-minute conversation cost
  • Requires real engineering; there is no visual flow builder

Which one fits

Choose Retell AI if you need

  • Teams replacing an outbound qualification or booking script
  • Contact centres that already have Twilio, Vonage or SIP trunking
  • Developers who want per-minute cost visibility before launch

Choose Ultravox.ai if you need

  • Developers building latency-sensitive voice agents
  • Teams that want the option to self-host the model later
  • Products needing SIP telephony alongside browser and mobile calls

More comparisons

All Ultravox.ai alternatives →Browse all Voice Agents & Phone AI →