In this blog, we are going to discuss Retell AI vs Synthflow and compare them across various areas like ease of use, voice customization, telephony setup, testing, integrations, concurrency, latency, and pricing to find out which platform is better for building voice AI agents.
Retell AI Overview
Retell AI is one of the easiest platforms to build voice agents on. From their platform, you can create an agent and choose a voice agent with a few options. The templates are very good..
You can build your voice agent out on a visual canvas. Instead of coding everything manually, you design the logic visually. You can enter your prompts into any of these nodes and use different node options such as call transfer, functions, conversation nodes, SMS, extracting variables, and MCP connections.
You enter in the prompt or a static sentence and then set up your transitions, which are the possible outcomes or the possible things the caller could say at that step. You connect them to the different nodes. If you click into any node, you have options for how to configure it, such as changing the LLM, adding knowledge, or making it a global node so the agent can jump to that node at any point in the conversation.
This setup also has some benefits from a latency perspective because rather than sticking one gigantic prompt that is being sent with every call to the LLM, you can break your prompt out into different steps, giving your voice agent only the context that it needs at each step of the conversation.
There are two different modes for building in Retell. You can have rigid mode where it follows the flow quite precisely, which is good if you have a very predictable conversational flow and want it to follow a script. Or you have flexible mode which allows for variability and gives the agent the autonomy to jump between different nodes when needed. It is visually very intuitive.
Explore more: Bland AI vs Vapi AI Comparison: Features, Performance, and Customization
Voice Engine and Customization
The default voice engine that Retell AI uses is ElevenLabs, which is generally recommended. You have other options like Cartisia, Miniax, and OpenAI voices. You can add your own custom voice by entering your voice ID and API key from ElevenLabs , or create a voice clone directly inside Retell AI by uploading audio files.
Telephony Setup
Retell AI makes it easy to connect phone numbers to your agent, and you can buy them from within the platform for $2 a month. If you are outside the US and Canada, you’ll need to import phone numbers via Twilio, Telnix, or SIP trunk. It takes a couple of minutes to get a phone number and verify it, and you can connect it to your agents for inbound or outbound calls.
Testing and Debugging Tools
One of the favorite features about Retell AI is testing. You can click the test agent and either chat with the agent or test the audio directly without needing it to call your mobile device. This makes it easier to test without racking up costs and credits. It also visually shows which step of the workflow is being followed. The voice quality is pretty good with different voice options available.
Integrations and Technical Flexibility
One weakness of Retell AI is integrations. If you are technical, you can connect it to anything using web hooks or model context protocol. However, for nontechnical users, options are limited. Retell AI does not have many native integrations. Cal.com is essentially the only native integration and works as a bridge between various calendar software like Google Calendar or Outlook via cal.com. For anything beyond calendar integration, you need to build a custom function or connect to an MCP server.
Concurrency and Scaling
Concurrency is important when building voice agents, especially for outbound calls or busy periods. By default, Retell AI allows up to 20 concurrent calls. It also has a concurrency burst feature where you can handle up to three times your normal limit, meaning up to 60 concurrent calls, at a slightly higher rate. If you consistently need more, you can adjust your limit at a cost of $8 per additional concurrent phone line.
Latency Transparency
Retell AI makes latency transparent. When building your voice agent, you can see the estimated latency in the top bar. Changing models or tiers can instantly reduce latency. In call logs, you can see actual latency and a breakdown of where it occurs, such as speech to text, LLM inference, or text to speech. This helps identify where improvements are needed. Average latency was around 714 to 756 milliseconds, which is very good and usable for production voice agents.
Pricing
In terms of cost, when adding LLM cost and telephony costs, Retell AI comes out around 13 cents per minute depending on the model and configuration. For a standard tech stack using GPT 4.1, deep transcription, and ElevenLabs Turbo V2, this is roughly accurate. It is not the cheapest option but is very reasonable. Retell AI gets testing, latency, and concurrency right and makes it easy to build production-quality voice agents, especially for non-technical users.
Synthflow Overview
Synthflow is a no code platform for building voice agents and has recently rolled out their flow workflow editor and overhauled pricing. Previously, building voice agents required adding everything into a single prompt, but now you can build agents in the flow designer. It is currently in beta and can be a bit buggy, but it provides a visual canvas similar to other tools with nodes, call transfers, conditionals, and actions.
Native Integrations Advantage
Synthflow stands out for native integrations. It integrates with Zappy and Mate.com, allowing connection to thousands of tools. It also has GoHighLevel integration and on cal.com for Google Calendar and some CRM integrations. This is helpful for non-technical users who do not want to use web hooks or MCP servers.
Telephony Setup
Inside synthflow , phone numbers cost $1.50 per month and can only be US, Canadian, or Australian numbers. If outside those countries, you need to import numbers via Twilio, Telnix, or Sip Trunk. A downside is that you cannot test agents directly on the platform without buying a phone number. You must either chat via text or have the agent call your phone number, though simulation is listed as coming soon.
Voice Engine and Customization
The default voice engine is ElevenLabs. You can import or clone voices. Voice configuration is pretty good though not as natural as some other tools, but there is significant control to tweak settings.
Concurrency Limits
By default, synthflow provides five concurrent phone lines. This is fine for most businesses but may be limiting for busy periods or large outbound campaigns.
Latency Concerns
In latency testing, synthflow had around 1 second latency, which was the slowest among popular tools. The platform does not clearly display latency information in analytics or call logs, which is a concern.
Pricing Model
Synthflow has moved from fixed monthly plans to pay as you go pricing. For around 3,000 minutes per month, cost would be approximately $423, depending on models used. It is generally not the most affordable option, but you are paying for no code convenience, pre-built integrations, and reduced build time.
Retell AI vs Synthflow
Retell AI vs Synthflow |
||
|---|---|---|
| Category | Retell AI | Synthflow |
| Voice Engine & Customization | Retell AI uses ElevenLabs by default and also supports Cartisia, Miniax, and OpenAI voices, along with custom voice IDs and voice cloning. | synthflow also uses ElevenLabs by default and allows voice import and cloning, with strong customization options. |
| Telephony Setup | Retell AI allows you to buy phone numbers for $2 per month and supports importing numbers through Twilio, Telnyx, or SIP trunk. | synthflow offers phone numbers for $1.50 per month but only for US, Canada, and Australia, with imports required for other regions. |
| Testing and Debugging | Retell AI allows in-platform testing through chat and direct audio testing without needing a phone call. | synthflow requires a connected phone number for live testing, although simulation is listed as coming soon. |
| Integrations and Flexibility | Retell AI has limited native integrations, mainly Cal.com, but supports webhooks and MCP for advanced connections. | synthflow stands out with strong native integrations including Zapier, Make, GoHighLevel, Cal.com, and CRM systems. |
| Concurrency and Scaling | Retell AI allows 20 concurrent calls by default with burst capacity up to 60 calls and scalable options. | synthflow provides 5 concurrent phone lines by default, which may limit large outbound campaigns. |
| Latency and Transparency | Retell AI shows estimated latency, provides detailed breakdowns in logs, and averages 714–756 ms response time. | synthflow averages around 1 second latency and does not clearly display latency breakdown information. |
FAQs about Retell AI vs synthflow
Which platform has better latency performance?
Average latency of Retell AI is around 714 to 756 milliseconds. It makes latency transparent. When building your voice agent, you can see the estimated latency in the top bar. Whereas synthflow has around 1 second latency, which is the slowest among popular tools. The platform does not clearly display latency information in analytics or call logs
Does Retell AI or synthflow offer better integrations?
Synthflow stands out for native integrations. It integrates with Zappy and Mate.com, allowing connection to thousands of tools. It also has GoHighLevel integration and on cal.com for Google Calendar and some CRM integrations. This is helpful for non-technical users who do not want to use web hooks or MCP servers. Whereas Retell AI does not have many native integrations. Cal.com is essentially the only native integration and works as a bridge between various calendar software like Google Calendar or Outlook via cal.com. For anything beyond calendar integration, you need to build a custom function or connect to an MCP server.
Which platform is better for scaling outbound campaigns?
Retell AI is better for scaling because it allows up to 20 concurrent calls. It also has a concurrency burst feature where you can handle up to three times your normal limit, meaning up to 60 concurrent calls, at a slightly higher rate. If you consistently need more, you can adjust your limit at a cost of $8 per additional concurrent phone line.
Which platform is more cost-effective for voice AI calls?
In terms of cost, synthflow has pay as you go pricing. For around 3,000 minutes per month, cost would be approximately $423, depending on models used whereas Retell AI comes out around 13 cents per minute depending on the model and configuration. So 3,000 minutes would cost about $390 at 13 cents per minute. So comparatively, Retell AI is cheaper and more affordable.
