Put a voice agent on a phone number in a few requests
Nixflex is a voice AI API for developers. Create an agent, attach your own number, place and receive calls, and get every call back as a webhook — all over one REST API, on your own carrier, for a flat $0.08 a minute.
POST /v1/agentsAuthorization: Bearer nxf_xxx:nxfs_xxx { "name": "Front desk", "system_prompt": "Greet callers and book appointments."}
One API, not a stack to assemble
Most voice tools ask you to wire a speech provider, a language model and a telephony vendor together yourself, then keep three integrations alive. Nixflex is a single REST API over an engine we run end to end. You send JSON and you get a working phone agent — no orchestration layer to build, no separate speech or model keys to manage, one bill instead of several.
The mental model is simple. An agent is one prompt and one personality. A phone number is a customer that uses that agent — and one agent can serve as many numbers as you like. Everything else — calls, SMS, integrations, analysis — hangs off those two objects.
What you can build through the API
/v1/agentsCreate, update, list and delete agents — the prompt, voice, language and behaviour of your voice AI.
/v1/phone-numbersImport your own Twilio or Telnyx numbers, attach them to agents, and manage their per-number settings.
/v1/calls/outboundPlace a single outbound call, or launch a batch campaign across a whole list of recipients.
/v1/callsPull call records with the full transcript, a summary, sentiment, and any structured data you asked the agent to extract.
/v1/smsSend text messages from your numbers, and run SMS campaigns — the same number handles voice and SMS.
/v1/keysCreate and rotate API keys, and read usage — revoke access without touching a single agent.
Authentication and control
Every request carries a bearer token in the form Authorization: Bearer nxf_xxx:nxfs_xxx — two halves joined by a colon. Keys are created and rotated through the API, so you can cut off access instantly without rebuilding anything. Because the platform is multi-tenant, you can run agents and numbers on behalf of each of your own customers under your account.
When an agent needs a real fact — a date, a callback number, an availability check — the engine supplies it rather than trusting the model to guess. Custom functions let the agent call your API mid-conversation and use the result to answer the caller, and every call fires a webhook the moment it ends, so your systems stay in sync without polling.
Why developers build on the Nixflex API
We run the whole voice pipeline, so latency and reliability are ours to own — and you can listen to a live call with Live Monitor.
Premium model included. No base fee plus per-minute plus separate model and speech charges to reconcile across vendors.
Bring your own Twilio or Telnyx account; your numbers and call costs stay on your bill.
Put a voice agent behind every one of your customers from day one — one API key, many numbers.
Receive every call as it ends, and let agents call your own APIs live during a conversation.
Structured errors and predictable shapes — the kind of API a sharp engineer can test hard and trust.
Evaluating APIs? Nixflex is a modern alternative to the Retell, Vapi and Bland APIs. Compare them →
Frequently asked questions
What is a voice AI API?
A voice AI API lets you build voice agents into your own product with code instead of a dashboard. You create an agent, attach a phone number, place or receive calls, and read the results — all over HTTP. Nixflex exposes this as one REST API, so a few requests put a live phone agent behind your app.
How do I authenticate?
Every request uses a bearer token made of two halves joined by a colon — Authorization: Bearer nxf_xxx:nxfs_xxx. You generate and rotate keys from the API, so you can revoke access without touching your agents.
What can I do through the API?
Create, update and list agents; import and manage your own phone numbers; place outbound calls and launch batch campaigns; send SMS; and pull call records with transcripts, summaries and sentiment. Custom functions let an agent call your own API mid-conversation, and webhooks push every call to your systems as it finishes.
Can I bring my own phone numbers?
Yes — you connect your own Twilio or Telnyx account and attach those numbers to agents through the API. Your numbers and carrier billing stay on your account, and Nixflex is multi-tenant, so you can manage numbers for each of your own customers.
How do I get call data back?
Two ways: pull it with GET /v1/calls, which returns records with the transcript, summary, sentiment and extracted data; or receive a webhook the moment a call ends, so the data lands in your own systems without polling.
What does the API cost?
Voice is a flat $0.08 per minute, pay-as-you-go, with the premium model included — not billed separately or requiring your own model key. Numbers with Live Monitor enabled bill inbound minutes at $0.09. SMS is billed per message at your carrier rate, and there is no monthly platform fee to start.
Start building with the voice AI API
Grab an API key and create your first agent in a few requests.