Nixflex - AI Voice Platform for Building AI Phone Agents

Nixflex is an AI voice platform for building real-time AI phone agents. You connect your Twilio or Telnyx number, give the agent a prompt, and your phone agent is live. Inbound and outbound calls, SMS, transfers, bookings, post-call analysis, and batch campaigns - all behind one API. Built for developers embedding voice agents into SaaS apps - and for businesses that want a phone agent without the plumbing.

Choose your path

I'm a developer

Use the API. Create agents, connect numbers, and go live with a few requests. Start with the Quickstart.

I'm a business

No code needed. Set everything up in the dashboard in about five minutes. Start with For businesses, or browse the Guides.

Why Nixflex

Real-time responses

The speech engine handles STT, the AI handles reasoning, and the voice engine speaks the reply - fast enough for a natural back-and-forth.

Bring your own carrier

Multi-tenant from day one. Multi-tenant from day one, on Twilio or Telnyx. Each customer connects their own carrier account - you keep full control of phone numbers and billing.

Live Monitor

Listen to a live AI call in real time from a phone or tablet. Because we run our own engine, we can tap the audio - something closed platforms cannot easily offer.

One number, voice + SMS

The same number is a call agent and a chat agent. When someone texts, your AI replies automatically - no separate chat bot to build.

Tag-based actions

The agent triggers transfers, SMS, and call endings through prompt tags. No webhook orchestration needed for built-in actions.

History-aware calls

Outbound calls automatically pull past call summaries for the recipient and inject them into the agent’s prompt. Your agent remembers customers across calls.

Your first agent

Create an AI phone agent with a single API call:

curl -X POST "https://api.nixflex.com/v1/agents" \
  -H "Authorization: Bearer nxf_xxx:nxfs_xxx" \
  -H "Content-Type: application/json" \
  -d '{
    "name": "Front desk",
    "system_prompt": "You are the friendly receptionist for Bella Hair Studio. Greet callers, answer questions, and book appointments."
  }'

Then attach a phone number, and the agent answers calls and texts on it. See the quickstart for the full flow.

Get started

Quickstart

Make your first call in under five minutes.

Authentication

Generate API keys and authenticate requests.

Create an agent

Configure prompts, voice, language, and actions.

API reference

Every endpoint with parameters, examples, and error codes.

How it works

A call moves through the engine like this:

Caller dials your number (or you trigger an outbound call via API).
Your carrier streams the audio to Nixflex over WebSocket.
Speech-to-text converts what the caller says into text in real time.
The agent reasons over the conversation and decides what to say or do next.
Text-to-speech turns the reply into natural voice and streams it back to the caller.
After the call ends, Nixflex runs post-call analysis (summary, sentiment, custom data extraction) and fires your webhook.

You configure the agent’s behaviour with a system prompt and a handful of settings. Everything else - audio handling, interruption detection, language matching, voicemail filtering - is handled by the engine.

Built-in capabilities

Inbound + outbound calls

Cold and warm transfers

Mid-call and post-call SMS

AI SMS replies (chat agent)

Live call monitoring

Voicemail detection

Batch voice + SMS campaigns

15 languages, with automatic switching across ten of them

Post-call summary, sentiment, data extraction

Custom functions (tools the agent can call mid-call)

Integrations

Connect your agents to the tools you already use - calendars, CRMs, messaging, and automation - configured per phone number.

Integrations overview

Cal.com, Google Calendar, Slack, GoHighLevel, Zapier, Make, and webhooks.

Bookings during a call

The agent checks availability and books live while the caller is on the line.

Pricing

Pay-as-you-go from $0.08 per minute of voice. SMS billed per message at your carrier’s rate. Numbers with Live Monitor enabled bill inbound minutes at $0.09. See pricing details for plans and volume discounts.

Need help?

Email support@nixflex.com or check the error reference for common issues.

​Choose your path