Choose Synthflow if you run a contact centre, BPO or many locations on a phone system such as Zoom or Five9 and want the vendor's engineers to lead the rollout. Choose Retell AI if your own team wants to build, test and change the agent, starting with a free trial. Treat both latency claims as marketing until you've heard your own calls.
Look at the main button on each home page and you've got most of the answer. One says book a demo. The other says try for free. That small difference tells you who each vendor expects to be talking to, how a project starts and who'll be making changes six months later.
For the rest of this page I'll call the first one the enterprise rollout and the second one the self-serve studio, because that's what each home page describes.
The short advice first
If you run a contact centre, a BPO or a brand with many locations, and your calls already flow through Zoom, Five9, RingCentral or a similar system, the enterprise rollout is built for you. Its engineers map your calls, connect your systems and launch where your agreement includes that support. If you'd rather have your own people build the call flow, run tests and change things whenever they like, the self-serve studio is the better shape, and you can start with a free trial before talking to sales.
Demo call or sign-up form: two ways in
Here's what surprises people who expect two similar builders. The enterprise rollout's own home page says it has no free plan or trial. Getting started means booking a demo and going through a sales-led process. It speaks to contact centres, BPOs and multi-location enterprises, and that's who its customer stories are about.
The self-serve studio invites you to try it free, offers a free live demo call so you can hear an agent, and keeps a Contact Sales route for larger teams. It pitches itself as a platform your team can run without relying on the vendor for every change.
Neither route is better in general. A sales-led rollout with engineers attached suits a business that wants someone accountable for the launch. A self-serve start suits a team that wants to experiment this afternoon. Just don't expect to sign up and poke around the enterprise product on your own.
Seven rows on how they differ
| Area | Synthflow | Retell AI |
|---|---|---|
| Getting started | Book a demo; the home page says no free plan or trial | Free trial, free live demo call and Contact Sales |
| Who runs the agent | Forward-deployed engineers map calls, connect systems and launch, where the agreement includes it | Your team, without relying on the vendor for each change |
| Building | Enterprise rollout; a customer story mentions a pilot in about five weeks | Drag-and-drop call flow canvas with functions, DTMF, SMS and custom logic |
| Phone systems | Zoom, Five9, RingCentral, 8x8, Twilio, Telnyx, Vonage, Freshcaller and its own telephony; Genesys via Enterprise setup | Twilio and Vonage, plus Make, Go High Level, n8n and HubSpot; branded caller ID and verified numbers |
| Channels | Phone first; SMS and WhatsApp in US workspaces only; web chat everywhere | Inbound, outbound campaigns, batch calling, web widget and SMS |
| Quality | Full transcript on warm transfer and call-level SIP visibility | Scenario simulation, production calls turned into regression tests, A/B analytics |
| Speed (vendor claim) | Replies in under half a second | Roughly 600ms latency |
Phone systems, numbers and channels
A contact centre can't rip out its phone system to try a new agent. That's where the enterprise rollout puts its weight: built-in connections for Zoom, Five9, RingCentral, 8x8, Twilio, Telnyx, Vonage and Freshcaller, plus its own telephony with a visible SIP call ladder. Genesys is handled through an Enterprise setup. Its site also says agents update the CRM during the call.
The self-serve studio's integrations lean towards automation tools and smaller business stacks: Make, n8n, Go High Level and HubSpot sit next to Twilio and Vonage. It also lists branded caller ID and verified numbers, which matter if your outbound calls keep getting ignored as unknown numbers.
Channels split in a way that could decide it for some readers. The studio covers inbound calls, outbound campaigns, batch calling, a web widget and SMS. The enterprise product is phone first, with web chat in all regions, but its SMS and WhatsApp agents are for US workspaces only. If you're outside the US and text matters, check that before the demo.
Checking quality before customers hear it
This is where the studio pulls ahead on paper. It says you can simulate thousands of scenarios, turn real production calls into regression tests and run A/B analytics on agent versions. It also claims QA across up to 100 percent of AI calls. If your team plans to keep changing prompts and flows, that kind of testing stops a fix in one place breaking something elsewhere.
The enterprise rollout talks about visibility instead: a full transcript passed over on warm transfer, so the human picking up knows what's been said, and call-level SIP detail for your telephony team. That's less about iterating and more about running a live operation calmly.
Whose customers vouch for them
The enterprise side names Freshworks, Thryv and Batteries Plus, the last one across 700+ Zoom Phone locations. Those are multi-site, phone-heavy businesses, which fits its pitch. The studio's case studies come from Pine Park Health, SWTCH and Medical Data Systems. If you want a broader view of tools in this space, our list of the best AI call centre software puts both in a wider field.
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Three other voice agent vendors
If neither shape fits, our page of alternatives to the self-serve studio goes further, and these three are the closest names.
Vapi
Best for: Developers who want a configurable, API-native voice agent platform to build, deploy and improve agents.
Bland AI
Best for: Healthcare, insurance and financial services call centres wanting phone, SMS and web agents; the vendor says 230+ regulated companies run on it.
ElevenLabs Conversational AI
Best for: Teams wanting voice and chat agents in dozens of languages from a vendor known for voice quality.
What to ask on your first call with either
- Who changes the agent after launch, and how long does a change take?
- Which of your phone systems connect out of the box, and which need extra setup?
- Do the channels you need, such as SMS or WhatsApp, work in your region?
- How can you test a new version against real calls before it goes live?
- Can you hear the agent on your own call scripts, not a polished demo?
Do you make an AI voice agent platform? Tool makers can ask to be considered for a featured place on this page.
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Frequently asked questions
Does Synthflow have a free plan or trial?
Its home page says it doesn't. You start by booking a demo, and the product is aimed at contact centres, BPOs and multi-location enterprises.
Can I try Retell AI before talking to sales?
Yes, it states a free trial and offers a free live demo call, with Contact Sales as a separate route.
Which one works with Five9 or Zoom Phone?
The enterprise rollout lists built-in connections for Zoom and Five9, among others. The studio's listed telephony integrations include Twilio and Vonage.
Which one is faster on a call?
One claims replies in under half a second and the other roughly 600ms. Both are vendor claims, so judge on your own test calls.
Can either send SMS or WhatsApp messages?
The studio covers SMS alongside calls and a web widget. The enterprise product offers SMS and WhatsApp agents in US workspaces only.
