Sonex Labs vs Retell AI: builder, cost and language coverage compared (2026)

Why teams compare Sonex Labs and Retell AI

Sonex Labs and Retell AI both sell a complete phone voice agent rather than a set of parts, so the choice between them is less about architecture and more about fit. Both bill per minute, both handle speech recognition, voice and telephony for you, and both let you bring your own language model. The differences show up in four places: how much of the bill sits outside the headline rate, whether the languages and carriers you need are covered, how many calls you can run at once, and how the agent is built and maintained.

It is worth deciding up front who will own the agent after launch. If an engineering team will own it through an API, the per-minute rate, language coverage and control over the model matter most. If an operations team will edit call flows every week, the builder carries more weight.

What are Sonex Labs and Retell AI?

Sonex Labs is a voice agent platform (Sonex Flows) that bundles speech to text, text to speech and telephony on Sonex numbers into one per-minute rate: $0.055 pay as you go, or $0.050 on a $5/month plan, as listed on our pricing page. The language model is billed separately: bring your own LLM key at zero markup, or use our Vyas LLM at $0.30 per million input tokens and $0.60 per million output tokens ($0.20 and $0.40 on the monthly plan). Our text to speech covers 250+ languages, including 36+ Indian languages and dialects, and our speech to text covers 100+. Agents answer in about 300 ms end to end, and every language page under text to speech has a live playground.

Retell AI is a voice agent platform built for teams that want to design, test and deploy phone agents with little code. Its pricing page (checked 2026-10-24) lists voice agents at $0.07 to $0.31 per minute, made up of $0.055 for voice infrastructure, a per-minute charge for the language model you pick, a text-to-speech charge and $0.015 per minute for telephony. Pay-as-you-go accounts start with $10 in credit.

What should you weigh when choosing?

The first factor is cost at your volume, which means the full per-minute figure including the language model, the voice you choose and any add-ons, not the infrastructure rate on its own. The second is coverage: the languages your callers speak, including mid-sentence switching, and whether you can use your own carrier and local numbers in the countries you call. The third is concurrency, the number of calls that can run at the same moment, which outbound campaigns hit quickly.

The fourth factor is who builds and maintains the agent: an API-first approach suits teams who version their agents like code, while a visual canvas lowers the skill needed to change a flow. The fifth is operational assurance: integrations, compliance documents, support terms and deployment options. Vendors measure latency differently, so treat published figures as a reason to test.

Sonex Labs and Retell AI at a glance

What does a minute of calling really cost?

Our rate already includes speech to text, text to speech and telephony on Sonex numbers, so the variable left is the language model. If you bring your own telephony, no minute charge applies to it and your carrier bills you. Speech to text is billed by the second, as described on our pricing page.

Retell's pricing page lists its components separately. Voice infrastructure is $0.055 per minute. Most voices, including Retell's platform voices, are $0.015 per minute, while ElevenLabs voices run from $0.04 (Flash) to $0.10 per minute. Telephony is $0.015 per minute, and custom telephony over your own SIP trunk carries no Retell charge. The language model is priced per minute by model, from $0.0016 for GPT 5 nano to $0.16 for GPT 5.5 at standard speed, with speech-to-speech models from $0.07 to $0.38. Optional add-ons include a knowledge base at $0.005 per minute, PII removal at $0.01 per minute, batch calling at $0.005 per dial and branded caller ID at $0.10 per outbound call.

The worked examples use these assumptions. Calls are outbound, average three minutes, and every connected minute is billed. Calls run in an eight-hour window across 22 working days, which puts average concurrency near one call at 10,000 minutes and near ten at 100,000 minutes; we plan for peaks of 3 and 20. The Retell stack uses its $0.015 platform voices and $0.015 telephony, one Retell phone number at $2 a month, and no add-ons. Sonex is on the $5 monthly plan, with the fee added on top of $0.050 a minute. For the language model we show GPT 5.4 mini at $0.024 per minute from Retell's list, a mid-priced option, separately. For Vyas LLM we assume 8,000 input tokens and 200 output tokens per call minute (about four turns with a 2,000-token context), roughly $0.0017 per minute at the monthly-plan rates. Your prompt size will change that, and bringing your own key replaces it with your provider's bill.

In the 10,000-minute scenario, the bundled rate comes to about $505 against about $852 on Retell before the language model, a difference of about $350. The model line moves that gap: a cheaper model on Retell such as GPT 5 mini at $0.008 per minute narrows it, and a premium voice widens it. The Sonex figure stays the same whichever voice you pick, which keeps cost predictable at volume. At 100,000 minutes the line to settle is concurrency: Retell includes 20 concurrent calls with each extra line at a published $8 a month, while we list platform capacity rather than a per-account price, so get your limit in writing before comparing totals.

Which speaks your customers' languages?

We publish support for 250+ languages in text to speech, including 36+ Indian languages and dialects such as Bhojpuri, Marwari, Garhwali and Maithili, and 100+ in speech to text, with automatic language detection, at one rate per language. For Arabic, Pāṇini covers Modern Standard Arabic and nine dialects: Gulf, Egyptian, Levantine, Najdi, Iraqi, Moroccan, Algerian, Tunisian and Sudanese. Mixed text such as Hinglish is read naturally in one voice. Any language can be heard before you build, on its live playground under text to speech.

Retell's language support page lists Hindi across several speech recognition and voice providers, Tamil, Marathi, Kannada and Urdu through Azure and Soniox for recognition, and Arabic across most providers. Telugu, Bengali, Malayalam, Gujarati and Punjabi do not appear on that page as of 24 October 2026. For callers who switch languages, Retell documents code-switching through Deepgram, Soniox and AssemblyAI, and its dashboard blocks multilingual setups where no single provider covers every language you have selected.

Can you call Indian numbers on your own carrier?

Sonex Labs includes telephony on Sonex numbers in the agent price, and you can bring your own telephony with no minute charge from us, so you keep the numbers and carrier contracts you already have. Our article on TRAI robocall rules covers the calling-hour and consent requirements for outbound calls in India. Sonex outbound campaigns take calling windows, time zones and retry limits, so calls go out only in the hours you set and stop after the number of attempts you choose.

Retell's pricing page lists India among the countries available through Twilio and Telnyx at the displayed $0.015 per minute, without separate per-country rates, and its custom telephony guide covers elastic SIP trunking and imported Twilio, Telnyx and Vonage numbers. Plivo's Retell guide notes that TRAI rules require voice AI platforms to terminate SIP traffic on servers in India and advises confirming with Retell before using Indian numbers. We found no Exotel-specific Retell documentation.

Who builds and edits the agent?

Sonex Flows agents are configured through the API described in our developer docs, so teams that keep agent definitions in version control and deploy them through an API can treat agents like the rest of their code.

Retell offers a drag-and-drop canvas where conversation flow agents split a call into nodes, each with its own instructions or actions, joined by transition conditions. For a non-developer owner editing scripted flows, that builder is its main advantage.

Will it connect to the systems you already use?

Sonex agents connect to HubSpot, Salesforce, n8n, Zapier and Make, support MCP and tool calling for your own systems, and can transfer a live call to a human agent. Calls can be recorded and transcribed. For other systems, such as a help desk or a clinical record, ask us before you build.

Retell publishes a longer integrations list. Native integrations include Zoho CRM, GoHighLevel, Cal.com, Calendly and Zendesk, and Five9 through a prebuilt SIP trunk. The same page documents routes to other systems: Intercom through an automation platform, Talkdesk, NICE CXone and Amazon Connect through SIP, and healthcare systems such as Epic, Dentrix and Open Dental through custom functions.

What do you get on latency, compliance and support?

We publish about 300 ms end to end, from when the caller stops speaking to when the agent starts, and Retell's homepage reports latency of about 600 ms. Both are vendor-reported and measured differently, and the number you experience will depend on your model, prompt length and carrier. Our platform capacity is 10,000 concurrent sessions; ask for your account's limit.

Retell's homepage states compliance with HIPAA, SOC 2 Type II, GDPR and ISO 27001, and its pricing page lists PII redaction, custom data retention, SSO and BAA options, and a dedicated stable server as an enterprise add-on. If a SOC 2 report or BAA is a hard requirement, ask both vendors for the documents and compare them side by side.

When is Sonex Labs the better choice?

Choose Sonex Labs if you run collections, EMI reminders or renewal calls in regional languages, where the caller needs to hear Telugu, Bengali, Tamil, Marathi or Hindi, or a dialect such as Bhojpuri or Marwari, including languages Retell does not currently document, or Arabic dialects such as Gulf, Egyptian and Levantine. Choose it if your numbers already live with a carrier: bring your own telephony and pay no minute charge to us for it.

Choose it if you want predictable cost at volume, with speech, voice and telephony in one rate, about $505 for 10,000 minutes in the worked example above, and the language model on your own key at zero markup or on Vyas LLM. Choose it if your CRM is HubSpot or Salesforce, or your workflows run on n8n, Zapier or Make. If you expect many simultaneous calls, get your account's concurrency limit in writing up front.

When is Retell AI the better choice?

Choose Retell AI if a non-developer will build and edit your agent on a visual canvas, you need 10 to 20 simultaneous calls from the first month at a published price, or your help desk or clinical system is already on its integrations list. It also fits if published HIPAA, SOC 2 Type II and ISO 27001 claims will speed your procurement review.

How to test both before you commit

Build the same agent on both, with the same prompt, the same tools and as close to the same language model as each allows. Ask the person who will maintain the agent to make one real change on each platform and time it. Use production telephony, meaning a local number in the country you will call, and place at least 50 calls per platform from real mobile phones across every language you need, including callers who switch languages. Measure latency at the handset, from the end of the caller's speech to the start of the reply. Score the calls on names, phone numbers, dates and amounts. Then compare a week of real invoices, including concurrency, against the worked examples above.