Stammer AI vs Retell AI: Voice Agent Platforms Compared (2026)

If you are evaluating voice AI platforms, Stammer AI and Retell AI both show up on most shortlists. Both build voice agents that handle calls. Both let you choose your underlying LLM. Both integrate with the usual stack of CRMs and automation tools.

But these two platforms are built for different businesses, priced for different P&Ls, and optimized for different outcomes. This article walks through what each one actually does well, where each one falls short, and how to tell which one fits your situation.

Quick Verdict

Pick Stammer AI if you are an agency, consultant, or SaaS operator who wants to sell AI voice and chat agents to clients under your own brand. The platform is built around reseller economics: client sub-accounts, full white-labeling, and pricing freedom on the markup you charge.

Pick Retell AI if you are a single business deploying voice agents in your own operations, especially in regulated industries or large call centers. The platform is built around production voice infrastructure: low-latency calls, deep telephony integrations, and enterprise compliance.

If you are still not sure which describes you, the rest of the article will help you decide.

At-a-Glance Comparison

Voice Agent Capabilities

Both platforms produce voice agents that can hold a real conversation, qualify leads, book appointments, look up information from a knowledge base, and transfer to a human when needed. The difference is in the technical posture each one takes.

Retell publishes specific latency numbers: roughly 600 milliseconds end-to-end, which they back with independent benchmarks. They also market a proprietary turn-taking model, which is the part of voice AI that decides when the agent should stop talking and start listening. Turn-taking is one of the harder problems in real-time voice, and Retell treats it as a key differentiator.

On model selection, both platforms give you a wide menu. Retell currently offers GPT 5, GPT 5.1, GPT 5.2, GPT 5.4, the GPT 4.1 family, Claude 4.5 and 4.6 Sonnet, Claude 4.5 Haiku, Gemini 3.0 Flash, and Gemini 2.5 Flash variants. Stammer supports GPT 4.1 nano, GPT 4o, GPT 4o mini, GPT 5, Claude 3.7 Sonnet, and Grok 2. Stammer also lets you bring your own OpenAI API keys for chat usage, which can cut your costs significantly if you have direct OpenAI billing.

Concurrent call handling is where the platforms diverge sharply. Retell includes 20 concurrent calls for free on every account, with additional capacity at $8 per concurrent call per month. Stammer's tiers start at 3 concurrent calls on the Agency plan, 5 on the Full SaaS Mode plan, and 25 or more on Enterprise. If you are running outbound campaigns at scale, Retell's structure makes that cheaper and simpler.

The capability that Retell does not match is chat. Stammer treats chat and voice as equal first-class products. You can deploy AI chat agents to your website, WhatsApp, Instagram DM, Facebook Messenger, and via API, all from the same dashboard. Retell offers chat, but it is positioned as a companion to voice rather than a standalone product line.

Telephony, Integrations, and Channel Reach

Stammer's voice telephony runs on Twilio. From the AI Voice Dashboard, you either connect your existing Twilio account or purchase new phone numbers directly through Stammer's interface.

For most agency use cases (local businesses, home services, real estate, SMB receptionist deployments) that path is more than enough, and the setup is genuinely simple. Where Stammer reaches further is in chat channel coverage: native deployment to WhatsApp, Instagram DM, Facebook Messenger, web embed, and a rebrandable API. That channel mix tends to match what SMB clients actually ask for.

Retell's telephony surface is broader. The platform supports SIP trunking to any carrier, plus direct integrations with the heavyweight contact-center-as-a-service platforms: Five9, Genesys, Avaya, Amazon Connect, Telnyx, Twilio, and Vonage.

If you are already running a Five9 or Genesys call center and need AI agents to slot into existing routing, Retell is built for that conversation. Retell also offers Branded Call ID and Verified Phone Numbers, both of which reduce the chance of outbound calls being labeled as spam by carriers.

On the automation and CRM side, the two platforms overlap heavily. Both integrate with HubSpot, Zapier, Make, n8n, and Go High Level. Retell adds Salesforce and Cal.com to that list. Stammer's integrations cover the agency-favorite tools, with Go High Level being the standout for AI agency operators.

Pricing and Business Model

The pricing models reveal what each platform is actually built to be.

Retell charges $0 to start. Every account gets $10 in free credits, 20 concurrent calls free, and full platform access with no monthly minimum. Voice calls run between $0.07 and $0.31 per minute depending on which LLM, voice provider, and add-ons you choose.

As a reference point, a GPT 5 voice call with a Retell platform voice and Twilio telephony runs roughly $0.125 per minute. Add-ons like knowledge base ($0.005/min), PII removal ($0.01/min), advanced denoising ($0.005/min), and branded call ID ($0.10 per outbound call) stack on top.

Chat agent pricing runs from $0.001 per AI message (GPT 5 nano) up to $0.03 per message (Claude 4.5 Sonnet). There is also an Enterprise plan with custom pricing for volume, dedicated servers, and 24/7 support.

Stammer charges $197 per month for the Agency plan, $497 per month for the Full SaaS Mode plan, and custom pricing for Enterprise. The subscription includes a fixed number of agents, knowledge base capacity, and concurrent calls per tier.

On top of the subscription, you pay usage: voice calls run from $0.11 per minute (GPT 4.1 nano) up to $0.17 per minute (Claude 3.7 Sonnet), and chat messages run from $0.001 to $0.03 each depending on the model.

The structural difference matters more than the per-minute numbers. Retell prices like infrastructure: you pay only when you use it, and the platform's revenue scales directly with your call volume. That works cleanly for a single business with a predictable call pattern.

Stammer prices are like a reseller platform: you pay a flat monthly subscription that gives you the right to create unlimited client sub-accounts, each with its own prepaid wallet. Your clients fund their own wallets, you set the prices they pay for messages and minutes, and you keep 100% of the markup with no revenue share to Stammer. The Agency plan economics only make sense if you are reselling to multiple clients. If you are deploying for yourself with low volume, Retell's pay-as-you-go is cheaper.

White-Label and Reselling

If reselling is the goal, this is a short comparison.

Stammer's entire product is built around white-label. You get a custom domain (something like agents.youragency.com), full dashboard rebranding with your logo and colors, branded client portals, a rebrandable API, and sub-account architecture where each client has their own login and wallet. Clients never see Stammer's name anywhere. The platform also includes MasterChat, an internal AI agent layer for managing your own clients, and a marketplace where you can list pre-built agents.

Retell does not offer white-label. The platform is sold under Retell's brand, and clients you build agents for will see that. Retell does run a Certified Partner program and an affiliate program, but those are positioned for solution partners and referrers, not for resellers building their own SaaS on top of the platform.

For an agency that wants to productize AI services and own the client relationship end-to-end, this single difference usually settles the decision.

Ease of Use and Time to Launch

Both platforms emphasize fast setup, but they assume different starting points.

Stammer is built for non-technical operators. The dashboard is no-code from end to end: define your agent's name and personality, upload knowledge base content, configure call flows, set up integrations, and launch. The onboarding includes a free AI agency course, daily office hours (Monday through Friday at 11:30am PST), and documentation aimed at agency owners rather than developers.

Retell is built for teams with some technical comfort. The platform offers a drag-and-drop agentic framework with built-in guardrails, real-time function calling, pre-built agent templates, and a simulation testing environment where you can validate agent behavior across scenarios before going live. The simulation testing is genuinely useful and rare among voice AI platforms. Retell also publishes thorough developer documentation and offers FDE-led (forward deployed engineer) implementation support on Enterprise plans.

If you are an agency operator without an engineering team, Stammer's learning curve is gentler. If you have technical staff and want to stress-test agent behavior before deploying to a regulated environment, Retell's tooling is more thorough.

FAQ

Can I white-label Retell AI?

No. Retell does not offer white-label functionality. The platform operates under Retell's brand. If white-labeling is a requirement, Stammer is the available option between these two.

Can Stammer handle outbound call campaigns?

Yes, Stammer supports outbound voice campaigns. Concurrent call limits depend on your plan tier (3 on Agency, 5 on Full SaaS Mode, 25+ on Enterprise). For high-volume outbound (thousands of concurrent calls), Retell's batch calling with no concurrency cap is more suited.

Do I need my own Twilio account to use either platform?

For Stammer voice agents, telephony runs through Twilio either way: you can connect your existing Twilio account or purchase phone numbers directly through Stammer's dashboard. Retell can provision Retell phone numbers for you at $2 per month per number (routed through Twilio or Telnyx) or accept your own carrier via SIP trunking

What happens to my agents and data if I switch platforms?

Knowledge base content and prompts are portable in both cases (you can export your training data). Call recordings, transcripts, and analytics history are platform-specific and do not migrate. Phone numbers can be ported between carriers but the process takes time. If platform-switching risk concerns you, factor it into your evaluation early.

Closing

In conclusion, Stammer and Retell are not really competing for the same buyer. If you are building an agency, Stammer's reseller architecture is the deciding factor. If you are running call operations for a single business in a regulated industry, Retell's compliance and telephony depth are the deciding factors.

If the agency model fits, you can start a free 14-day Stammer trial with no credit card.