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This guide takes you from zero to a working AI agent answering and making real phone calls. You’ll need:

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Step 1 - Get your API key

1

Sign in to the dashboard

Open dashboard.nixflex.com and sign in. An API key is created for your account automatically.
2

Reveal your key

Go to API Keys, click Reveal, and copy the full credential. It looks like:
nxf_a1b2c3...:nxfs_x1y2z3...
3

Use it in requests

Authenticate every API call with:
Authorization: Bearer nxf_xxx:nxfs_xxx
Treat your full credential like a password. Never commit it to source control or expose it in client-side code. If it leaks, rotate immediately from the dashboard.

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Step 2 - Create an agent

An agent is the AI personality that answers calls. Each agent has a prompt, voice, language, and behaviour settings. 
curl -X POST https://api.nixflex.com/v1/agents \
  -H "Authorization: Bearer YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "name": "Test Receptionist",
    "system_prompt": "You are a friendly receptionist for Acme Dental. Greet callers, answer questions about opening hours (Mon-Fri 9am-5pm), and book appointments by collecting name and preferred date.",
    "welcome_message": "Hello, Acme Dental, how can I help?"
  }'
The response contains your agent_id: 
{
  "agent_id": "agent_125207e452f8714a",
  "name": "Test Receptionist",
  "voice_id": "Ashley",
  "language": "multi",
  "is_active": true
}
Save the agent_id - you’ll attach a phone number to it next.
The default voice is Ashley and the default language is multi (auto-detects 15+ languages including Arabic, French, Spanish, German). Change them later via the dashboard or the Create agent reference.

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Step 3 - Connect a Twilio number

Nixflex uses a BYO Twilio model. You import a number you already own in Twilio, and Nixflex configures the webhooks automatically. 
curl -X POST https://api.nixflex.com/v1/phone-numbers \
  -H "Authorization: Bearer YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "phone_number": "+447446466847",
    "twilio_sid": "ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
    "twilio_token": "your_twilio_auth_token",
    "agent_id": "agent_125207e452f8714a"
  }'
Nixflex points all webhooks on this Twilio number at the engine. The agent now answers when this number rings, and can use it to make outbound calls.

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Step 4 - Test inbound

Call your Twilio number from any phone. The agent will:
  1. Pick up
  2. Say your welcome message
  3. Listen, respond, handle interruptions naturally
  4. Hang up when the conversation ends or when the caller stops talking

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Step 5 - Test outbound

Have the agent call you. Replace the to_number with your mobile. 
curl -X POST https://api.nixflex.com/v1/calls/outbound \
  -H "Authorization: Bearer YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "agent_id": "agent_125207e452f8714a",
    "to_number": "+44YOUR_MOBILE"
  }'
Your phone rings within seconds. Pick up and talk to your agent.
You don’t pass a from_number. The engine uses the Twilio number you assigned to the agent in Step 3.

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Step 6 - Read the call back

After the call ends, pull the details: 
curl https://api.nixflex.com/v1/calls?limit=1 \
  -H "Authorization: Bearer YOUR_KEY"
The response includes the transcript, recording URL, post-call summary, caller sentiment, and any custom data the agent extracted.

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What’s next

Outbound calls

Trigger AI calls programmatically with one API request.

Agent actions

Let your agent transfer calls, send SMS, and end calls using prompt tags.

Webhooks

Receive post-call data in your own systems automatically.

Custom functions

Let the agent call your APIs mid-call (lookup, booking, payments).