Sierra or Retell AI: What I'd Check Before Buying a Voice Agent Platform

Here's where I'm coming from. Sierra markets itself as a premium, white glove enterprise solution. On chat and omnichannel, I think that reputation is absolutely earned. On voice specifically, I don't think the product matches the premium. Rather than just tell you that, I tested it. And for full transparency, Retell is the platform I build my clients' agents on.

The call test

I called Sonos, which runs a real Sierra voice agent, and told it my speaker stopped working. It understood me fine. It asked which model I had, then whether the speaker was shaped more like a cylinder or a rectangle. But the pauses were so long I had to hang up. The intelligence is there, but man.

Then I called a Retell equivalent with the same problem. It asked which product, offered Move or Roam, asked what the glitch was, and started walking me through checking the volume. Quick back and forth, the way a person would do it.

That's not a comprehensive test by any means. But at face value you can already hear clear differences.

Speed is the whole game on the phone

Two humans talking on the phone run around 500 milliseconds between turns. Retell advertises around 620. In reality it's a little higher, but in the best conditions that's what you get. Sierra came in around 1,400 milliseconds at baseline in our own tests, and on that live Sonos call it was a lot worse than that.

I'm not saying Sierra is bad. I'm saying Retell is abnormally good on latency. On an independent voice AI benchmark plotting platforms by performance and latency, Retell came out on top.

If your customers reach you by phone, this isn't a technical detail. Every pause is something the caller sits through.

Who owns the agent when it's done

Both companies will build the agent for you. What happens afterward is not the same.

Retell has forward deployed engineers who'll build the whole thing white glove, like they've done for Lenovo and Motorola. Then, if you want, they transfer full ownership to your own development team to maintain. And because it's an open platform, you can keep customizing it to your exact use case.

Sierra, from what I can tell, is white glove only. The agent lives permanently on their closed platform and they retain 100% ownership. That's vendor lock in, and it's worth knowing before you sign anything.

Who controls your data

Both are SOC 2 certified and both cover HIPAA. The difference is who holds the keys. Sierra runs a managed vault. They hold the audio, the guardrails, and the compliance. In one sense that's peace of mind because they take care of it. In another, it's less control.

Retell works more like a programmable pipe, and it's one of my favorite things about it. There's literally a button to redact personal info like Social Security numbers, credit cards, and passport numbers. If you're under PCI or HIPAA and you have a security team with opinions, controlling the pipe yourself is worth a lot. If you don't have that team, Sierra's managed vault might honestly be the better fit.

Price, and when each one makes sense

Retell posts pay as you go pricing, roughly 7 to 20 cents a minute depending on the model and setup. You can price it out with their calculator today. Sierra reportedly charges per resolution, around $1 to $2.50, and reports I've read put year one at $200k to $350k or more just to get a simple solution built. To find out for sure, you have to get on a sales call.

Sierra has a big name behind it in Bret Taylor, who sits on the board of OpenAI. That doesn't automatically mean a better product.

That said, there is a real space for Sierra. If your support mostly lives in a chat window and you want chat, email, and voice under one contract, the way a lot of ecommerce and software companies do, Sierra might actually be the better choice.

But if the phone IS the business, like healthcare reception, patient intake, or high call volume your team can't absorb, where a pause actually costs you, Retell has clear advantages I don't think Sierra can match. Especially at that price tag.