Comparing PolyAI and Sierra AI? Both are Enterprise 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 | PolyAI | Sierra AI |
|---|---|---|
| Pricing | Per resolution · Custom (per-minute) | Per resolution · Custom |
| Founded | 2017 | 2023 |
| Categories | Enterprise Voice & Phone AI | AI Agents & Chatbots Enterprise Voice & Phone AI |
| Integrations | Salesforce HubSpot Zendesk Microsoft Dynamics OpenTable Zapier | Salesforce Zendesk Shopify Twilio Stripe Snowflake |
PolyAI answers the phone. While most of this directory started with chat and added voice later, PolyAI has been voice-first since three Cambridge machine-learning PhDs, Nikola Mrkšić, Pei-Hao Su, and Tsung-Hsien Wen, founded it in London in 2017 out of the university's Machine Intelligence Lab. Its agents are known for sounding genuinely human and for surviving the things that break lesser systems: interruptions, accents, background noise, and callers who change their mind mid-sentence. Restaurants, hotels, banks, and utilities use it to answer every call instantly, day and night, with customers like PG&E, UniCredit, Golden Nugget, and Fogo de Chao on the roster.
The platform matured into what the company calls an agentic dialog platform: Agent Studio for building and managing agents without code, a developer kit for deeper work, and Raven, a proprietary conversation model trained on more than a billion enterprise conversations. Compliance boxes (SOC 2, HIPAA, PCI) are ticked, which matters because taking payments and health information over the phone is exactly where voice AI earns its keep.
Investors have noticed the wedge. In December 2025 PolyAI raised an $86 million Series D at a valuation around $750 million, with NVIDIA's venture arm participating, bringing total funding past $200 million. Commercially it bills on usage, priced per minute of conversation, with rates scoped to your call volume rather than published on the site.
Choose PolyAI when the phone is your problem. If most of your support pain is hold music, missed calls, and staffing the night shift, a voice-native specialist will beat a chat-first platform's bolted-on telephony, and PolyAI is one of the most proven voice specialists in the market.
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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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