Synthflow vs Voiceflow: Annual Contract or Self-Serve Platform

Synthflow and Voiceflow are both hosted, closed-source platforms. Synthflow is built for phone calls and sells one plan, Enterprise, with contracts from $30,000 a year. Voiceflow is a self-serve support platform for chat, phone and SMS, with a $0.05 phone minute plus credits. For code you can run yourself, there is Dograh, an open-source, self-hostable voice agent platform.

Key Takeaways

• Synthflow contracts start at $30,000 a year, while Voiceflow starts free, then bills plans and credits.
• Synthflow offers US and EU hosting, and Voiceflow is hosted, with private cloud on Enterprise.
• To run the platform on your own servers, self-host Dograh, which is open source.

This post is part of our guide to Voiceflow alternatives. The guide's section "Hosted voice platforms: Vapi, Retell, Bland, Synthflow" says Synthflow "suits larger teams that want a vendor to scope and launch the project with them." This post puts the two side by side on how each one is bought, plus every other point that changes a buying decision.

A sales-led voice platform and a self-serve one

Synthflow sells a voice project you scope with its sales team, while Voiceflow sells a support platform you can start on yourself. Most rows below follow from that.

How each one is bought, agencies included

Synthflow's bill starts at a fixed annual floor, while Voiceflow's grows with your plan, add-ons and the credits your calls use.

Synthflow's pricing page says "Enterprise contracts start at $30,000 annually," with the final price "scoped around call volume, concurrency, telephony setup, integrations, security needs, and launch support." The contract also buys implementation, onboarding, testing, training and launch support. Our post on Synthflow pricing explains what the contract covers.

Voiceflow starts with a Free plan whose credits do not renew, then Pro, Business and Enterprise. A bill is a monthly plan fee, add-ons such as editor seats and phone numbers, and credits. A phone minute is $0.05 on self-serve plans, and speech-to-text minutes, voice characters and model tokens each use credits on top. Voiceflow's pricing page offers businesses implementation support that is "self-serve or fully managed," with pricing on request. Our breakdown of Voiceflow pricing prices every line.

So a pilot on Voiceflow can start the same day on its own numbers, while on Synthflow it starts with a sales conversation and an annual commitment.

Agencies get different tools on each. Voiceflow lists a free trial with no credit card, multi-client workspace management, white-labeling and client handoff tools. Synthflow keeps subaccounts, permissions, branding and usage limits for clients, but its reselling notice says "The Stripe integration for reselling to subaccounts was removed on September 15, 2026." Agencies on Synthflow now invoice their clients outside the product.

Phone numbers, SIP and outbound calls

Synthflow connects to more phone systems out of the box, while Voiceflow's own numbers answer calls but can't place them.

Synthflow gives you three ways to connect a line. You can buy a number from Synthflow, use your own Twilio account, or connect your SIP trunk or PBX. Bought numbers cover the US, Canada and Australia. Its tested, native integrations include Twilio, Telnyx, RingCentral RingCX, Vonage, 8x8, Zoom and Five9, which matters for a contact centre that already runs one of them.

Voiceflow can provision one US or Canadian number per workspace with your plan, with more as an add-on, and you can connect your own Twilio, Vonage or Telnyx number. Voiceflow phone numbers currently don't support outbound calling, so outbound calls go out through your own carrier's number, one API request per call. To reach a person, Voiceflow's Call forward step transfers the caller to a phone number, an international number or a SIP address.

Building an agent: Flow Designer or playbooks

Synthflow draws a call as a flow of nodes, while Voiceflow builds around playbooks, workflows and ready-made support tools.

Synthflow's Flow Designer has nodes for greetings, conversation steps, messages, branches, jumps, custom actions, call transfer, real-time booking and ending the call. A Trigger Subflow node hands the call to a sub-agent and returns to the main flow automatically when it finishes. Synthflow's model list runs from GPT-4o to GPT-6-Luna, plus its own Synthflow LLM, and "GPT and other third-party models are only available on Enterprise plans," which is the only plan on sale.

A Voiceflow agent combines a global prompt with playbooks, each pursuing one goal, and workflows for steps that must run the same way every time. Native Zendesk, Salesforce, HubSpot and Shopify tools come ready to use, and a traffic split sends a share of traffic to each version of the agent for A/B tests. For voice, you choose speech-to-text, the language model and the voice from Voiceflow's provider list, or bring your own model.

Open Source Alternative to Vapi / Retell

Self-hosted voice agent platform — no per-minute fees

dograh-hq/dograh

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HIPAA, data location and certificates

Synthflow writes its BAA into the Enterprise contract and lets you pick a US or EU region, while Voiceflow arranges its BAA through its security team and hosts the agent itself.

Synthflow signs its business associate agreement (BAA) at contract signing for customers using the HIPAA Setting, which is off by default. Voiceflow "supports HIPAA-compliant deployments," and asks you to contact its security team about the BAA for your use. Generally, whichever vendor signs, a BAA hands it obligations while your own HIPAA duties stay with you; confirm your position with counsel.

Synthflow runs on its own cloud, with region-based hosting in the US and EU. Synthflow's region docs describe its EU cluster as having "all data and processing fully in the European Union," and its US cluster the same for the United States. It is retiring its older Global cluster during Q4 2026. Voiceflow's pricing page says "All hosted on Voiceflow," and its billing page lists private cloud hosting on Enterprise.

For certificates, Synthflow's trust center lists SOC 2 Type 2, ISO/IEC 27001, PCI DSS v4.0.1, HIPAA and GDPR. Voiceflow lists SOC 2 Type II, ISO/IEC 27001:2022 and GDPR.

Synthflow vs Voiceflow: picking between them

Pick Synthflow if you want the vendor to build and launch a phone project under one contract, with EU or US processing and your existing contact-centre phone system. Pick Voiceflow if you want to start on your own today, especially when one agent has to answer on chat and phone or your support team relies on its native helpdesk and CRM tools.

Agencies should weigh Voiceflow's white-labeling and multi-client workspaces against Synthflow's subaccounts, now that Synthflow's in-product reselling is gone. Teams weighing another option can read Vapi vs Voiceflow, or Synthflow alternatives for the field from Synthflow's side.

Run the same call script on both over a real phone line before you sign. Most phone calls still reach the agent as 8 kHz audio, while a browser demo runs at 16 kHz, so a demo that sounds sharp can mishear callers on the phone.

Where an open-source platform fits

Both are hosted and closed source. Synthflow runs on its own cloud in the US or EU, and Voiceflow runs on its own, with private cloud on Enterprise. Neither lets you install the platform in a data centre you choose without a vendor contract, and that is where Dograh fits.

Dograh is open source under BSD-2-Clause, and its GitHub repository, with more than 5,900 stars, holds the whole platform. Its visual workflow builder joins nodes with pathways, each with a written condition the AI reads, so a team used to the Flow Designer will find it familiar. In Dograh you build a sub-flow with pathways: one pathway into that part of the workflow and one back to the next step. The return isn't automatic, so you draw it yourself.

Dograh is the orchestration layer: it runs the call flow, tools and telephony, and the speech and language models are a separate layer you choose. That can be Dograh's own models, any supported provider on your own key, or open-weight models you host. Self-hosted, Dograh runs where you install it, such as an EU data centre. The call audio stays in-house only when the speech and language models run on your own servers too.

Dograh starts every account on its own managed models. To keep call audio away from Dograh, open Models, switch the Organization default to BYOK with your own speech and language providers (or local models), and check that no agent has its own model override.

Your numbers stay with your carrier, such as Twilio, Vonage, Plivo or Telnyx. Transfer Call works on Twilio, Telnyx and Asterisk ARI. It passes no summary by default; on Twilio it can play a short spoken introduction to both people. Write the details to your system with an HTTP API tool before the transfer.

For client billing, Dograh connects to Paygent, which tracks each call's usage and invoices your clients at your own rates. Dograh is built for phone calls and also offers a text chat widget for websites, though teams whose main channel is text chat may prefer Voiceflow.

Self-hosted Dograh has no per-minute platform fee (servers, model keys and phone minutes still cost); Dograh Cloud has a 1¢-a-minute platform fee, with 10 concurrent calls included. Self-hosting puts uptime in your team's hands, and there is no one-click import from Synthflow or Voiceflow, so each agent is rebuilt.

Dograh is the strongest fit when you want to choose where the platform runs, with no contract floor, because the code is yours to run. To try it, self-host it with Docker Compose, or start on Dograh Cloud.