Comparing Retell AI and Sierra AI? Both are AI Agents & Chatbots and Voice & Phone AI tools in the directory, which is why buyers put them on the same shortlist. Below is a side-by-side look at how they price, what they integrate with, and when each is the better fit, so you can pick on the facts rather than either vendor's own sales page.
| Attribute | Retell AI | Sierra AI |
|---|---|---|
| Pricing | Paid · $0.07/min | Per resolution · Custom |
| Founded | 2023 | 2023 |
| Categories | AI Agents & Chatbots Voice & Phone AI | AI Agents & Chatbots Enterprise Voice & Phone AI |
| Integrations | Twilio Vonage Telnyx Cal.com n8n Make GoHighLevel custom SIP trunking | Salesforce Zendesk Shopify Twilio Stripe Snowflake |
Retell AI is a platform for building AI phone agents that answer and place calls. You compose an agent from interchangeable parts: a speech-to-text layer, your choice of LLM, a voice from providers like ElevenLabs, and telephony via Twilio, Vonage, Telnyx, or your own SIP trunk. The platform handles the hard real time bits, interruption handling, latency, call transfer, and IVR navigation, and adds testing, monitoring, and post call analysis aimed squarely at contact center and front desk workloads.
The company came out of Y Combinator's Winter 2024 batch, founded in 2023, with Evie Wang among the cofounders. It grew unusually fast for a seed stage company: a $4.6 million seed led by Alt Capital in September 2024 arrived alongside a reported $3 million in annual revenue within months of launch. Named customers include telehealth company Ro, plus BPOs and contact center operators.
Pricing is published and usage based, which is refreshingly rare. Retell quotes $0.07 to $0.31 per minute all in, depending on your voice and model choices, with components itemized: text-to-speech from $0.015 per minute, LLM costs from $0.003 to $0.16 per minute, US telephony around $0.015 per minute. Every account includes 20 concurrent calls, extra concurrency is $8 per line monthly, phone numbers cost $2 monthly, and new accounts get $10 in free credits. Enterprise plans with dedicated infrastructure are custom.
Choose Retell if you have some technical ability, want a working phone agent this week, and value published per minute economics you can model before committing. Its contact center tilt (batch calling, warm transfer, IVR traversal) makes it a natural fit for support lines. Look elsewhere if you want a fully managed enterprise deployment with a vendor doing the tuning, or a pure no-code experience for a non-technical team.
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Sierra builds conversational AI agents for the enterprise, and it arrived with about as much star power as a startup can carry. It was founded in 2023 by Bret Taylor and Clay Bavor, and if those names ring a bell it is because Taylor co-created Google Maps, served as CTO of Facebook and co-CEO of Salesforce, and currently chairs the board of OpenAI, while Bavor spent eighteen years at Google leading its virtual and augmented reality work. Company lore says the whole thing was hatched over a grilled branzino lunch in Palo Alto.
The product lives up to the pedigree. From a single build, Sierra deploys a branded agent across chat, voice, email, SMS, and even ChatGPT, following your policies, adopting your brand's voice, and escalating gracefully when a conversation gets thorny. Its voice agents are a standout: they hold natural, low-latency conversations, read sentiment, and can even take a credit card or ACH payment over the phone in one uninterrupted call, with no clumsy handoff to a separate system.
Under the hood sits Agent OS, a platform for building, testing, and supervising agents, with tooling to run voice simulations and rehearse behavior before anything reaches a real customer. Pricing is outcome-based, tied to conversations or successful resolutions and wrapped in multi-year enterprise contracts, so Sierra is aimed at large brands rather than weekend experimenters. Reported customers include SoFi, Ramp, and Brex, and the company says its agents already serve a meaningful slice of the Fortune 50.
Sierra's fundraising has kept pace with its towering ambition, vaulting the company into the rarefied tier of the most valuable AI startups in barely two years. For enterprises that want polished, on-brand agents across every channel and are willing to invest the time and budget to do it properly, Sierra is a genuine marquee choice.
Read the full Sierra AI listing → · See Sierra AI alternatives →
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