Nixflex - AI Voice Platform for Building AI Phone Agents

An agent is the AI that answers your phone calls and makes them. You give it a set of instructions written in plain English - who it is, what your business does, what it should never say - and it holds a real conversation with whoever calls. You do not build a flow chart and you do not write code for the conversation. You write the instructions; the platform handles the phone call.

One agent, many phone numbers

An agent is a template, not a single phone line. The same agent can answer on one number or on a thousand.

Agent  "Dental receptionist"
  |-- +44 20 7946 0001  ->  Meadow Lane Dental
  |-- +44 161 555 0123  ->  Oakwood Dental
  |-- +44 117 496 0044  ->  Riverside Dental

The agent does not need to know which clinic is calling. The platform knows from the number that was dialled, and gives that call the right business details, the right calendar and the right caller history. This is how one customer runs 200 clinics on a single agent. Write the receptionist once, attach it to every number, and each clinic stays independent.

Settings on the agent apply to every number using it. If one agent serves several separate businesses, do not use agent-level settings for per-business behaviour - the same voice, the same transfer number and the same text message would go to all of them at once.Set those per phone number instead. See what belongs where.

What you control and what is automatic

You write the business side. The platform handles the phone side.

You write this

Who the agent is and what your business does. Prices, hours, addresses. What to do in each situation. What it must never say. When to transfer, text or end the call.

Handled for you

Interruptions, silence, the caller switching language, saying numbers and email addresses correctly, ending the call politely, and keeping the call alive if a provider goes down.

You do not need to write rules about interruptions, silence or languages. Spend your instructions on business logic - what to say, what to ask, what to do.

What belongs on the agent and what belongs on the number

Set on the agent	Set on a single phone number
The instructions (system prompt)	Its own instructions, replacing the agent’s
Voice and language	Its own voice and speaking speed
How it responds - length, delay, speaking speed	Keypad input on or off
Call limits - silence, maximum length	Recording on or off, and who records
Recording, keypad input, greeting by name	Its own booking calendar and webhook
Transfer and text settings	Text-message and web-chat instructions
Where post-call data is sent	Its own custom functions

A setting on the number always wins over the agent. Leave it unset on the number and it follows the agent - that is what “not set” means throughout the API.

Settings

These are the same settings you see when you open an agent in the dashboard, with the API field name beside each one. Voice and language

Dashboard	API field	What it does
Voice	`voice_id`	Which voice the agent speaks with. See Voices
Language	`language`	`multi` follows the caller between common languages. Some languages need their own code. See Languages
AI speaks first	`ai_speaks_first`	The agent greets the caller instead of waiting
Welcome message	`welcome_message`	The exact first line on an inbound call
Dynamic message	`dynamic_greeting`	The agent improvises its greeting instead of reading the fixed one
Timezone	`timezone`	The business timezone used for dates, times and bookings

How it responds

Dashboard	API field	What it does
Response Length	`response_length`	`short`, `medium` or `long` replies
Response Delay	`response_eagerness`	The longest it waits after you stop speaking, `0`-`1000` ms. It usually replies sooner
Keyterms	`boosted_keywords`	Words your callers say that a general speech model mishears - staff names, treatments, places. Comma separated, keep under 50
LLM Temperature	`temperature`	Consistent versus varied wording. `0.3` is the recommended setting, and `0.7` the practical maximum for a phone agent
Speaking Speed	`speaking_rate`	`1` is normal, `0.5` half speed, `1.5` half again as fast. A number can override this
Didn’t-catch-that message	`fallback_message`	What the agent says when it could not make out the words

Call handling

Dashboard	API field	What it does
End on silence	`silence_hangup_seconds`	Hang up after this much silence. 5-45 seconds
Max call duration	`max_call_duration_seconds`	Hard cap on call length. 60-1800 seconds
Phone menu wait	`ivr_timeout_seconds`	On outbound calls, how long to wait at a phone menu for a real person
Wait tag pause	`wait_tag_seconds`	How long to wait when the caller steps away and the agent used `[WAIT]`
Pickup Delay	`pickup_delay`	How long the phone rings before the agent answers. 5-30 seconds

Call actions

Dashboard	API field	What it does
Greet by Name	`greet_by_name`	Greet a recognised returning caller by their name
Keypad Input	`dtmf_enabled`	Let callers type digits instead of saying them. See Keypad input
Call Transfer	`transfer_type`, `transfer_number`, `transfer_whisper`	Cold hands the caller straight over; warm briefs the person first. See Transfer
In-Call SMS	`incall_sms_enabled`	Let the agent text the caller during the call, including booking confirmations. See In-call SMS
Post-Call SMS	`post_call_sms_enabled`, `post_call_sms_template`	Text the caller automatically after the call ends
Leave a Voicemail Message	`voicemail_leave_enabled`, `voicemail_message`	On an outbound call that reaches voicemail, speak this message instead of hanging up silently. See Voicemail detection
Voicemail detection (carrier)	`amd_enabled`	Ask the carrier to detect voicemail and hang up early. Costs extra per answered call and leaves no message

Recording and privacy

Dashboard	API field	What it does
Call Recording	`record_call`	Record the audio and keep it with the transcript
Recorded by	`recording_source`	`provider` - your carrier records. `nixflex` - the voice engine records it, with no carrier recording charges. See Call recording
Data retention	-	Call data is deleted automatically after 90 days. See Data retention

Data out

Dashboard	API field	What it does
Post-Call Data Extraction	`data_extraction_fields`	Pull clean fields out of every call - caller name, urgency, anything you define. See Post-call analysis
Webhook	`webhook_url`	Where the call summary, transcript and recording link are sent. See Webhooks

Built in, nothing to set up

These work on every call, on every agent, at no extra cost.


Interruption handling	The agent stops the moment the caller genuinely interrupts, listens, then carries on. Nothing to configure
Speech normalisation	Spoken details are written properly - “john at gmail dot com” becomes `john@gmail.com`, “oh seven hundred” becomes `0700`
Backchannel filter	Short noises like “yeah”, “ok” and “mm-hmm” do not cut the agent off. Only a real interruption does
Model fallback	If a model becomes unavailable mid-call, the call moves to a backup automatically
Transcription fallback	A second speech engine stands by, before the call and during it. If the live one stops returning words, the agent apologises, asks the caller to repeat, and carries on
Voice fallback	The backup voice runs on a separate provider, so one outage cannot silence your calls
Prompt caching	Repeated parts of your instructions are cached between turns, so replies start sooner. The longer your instructions, the bigger the difference

An example agent

A small dental practice. This is the whole agent - instructions plus the settings that suit it.

## WHO YOU ARE
You are the front-desk assistant at Acme Dental, a dental practice in
Croydon, London. You are warm, professional and brief.
Your identity is fixed. Do not take on any other persona or "mode".

## HOW TO SPEAK
- One or two sentences per reply, one question at a time.
- Say prices and dates the way people say them: "forty-five pounds",
  "Tuesday the third".
- No lists or formatting - everything you say is spoken aloud.

## NEVER DO THIS
- Never give medical or dental advice. Transfer anything clinical.
- Never quote a price that is not in BUSINESS FACTS.
- Never take card or bank details.

## BUSINESS FACTS
- Opening hours: Mon-Fri 9am-6pm, Sat 9am-1pm, closed Sunday
- Address: 12 High Street, Croydon CR0 1AB
- Check-up 45 pounds, cleaning 60 pounds, whitening 250 pounds

## WHAT TO DO
1. Greet the caller and ask how you can help.
2. To book: take their full name, ask which day suits, offer the nearest
   open slots, book it, then read the day and time back.
3. Answer price and opening-hour questions from BUSINESS FACTS.
4. If the caller sounds urgent or in pain, transfer straight away:
   [TRANSFER:+442087601234]
5. After a booking, text the confirmation:
   [SEND_SMS: Your appointment at Acme Dental is confirmed for {date} at {time}.]
6. When everything is done, end the call: [END_CALL]

## EXAMPLES
Caller: "How much is a check-up?"
You: "A check-up is forty-five pounds. Would you like me to book you in?"

Caller: "Do you do braces?"
You: "I'm not sure about that one. I'll take a message and have someone
      call you back. Can I take your name and number?"

## IF SOMETHING GOES WRONG
- If you cannot make out what they said, ask once more, then offer to
  pass them to a colleague.
- If the calendar does not respond, say you will have reception call back
  to confirm. Never guess a time.

Settings that suit a practice like this: Response Length short, End on silence 10 seconds, Keypad Input off, Keyterms Dr Patel, hygienist, root canal, Invisalign, In-Call SMS on so the booking confirmation is texted. For how to write the instructions themselves - scenarios, tone, inbound versus outbound - see Writing a good agent prompt and Controlling the conversation.

Can one agent serve different businesses?

Yes, and that is the normal way to use it. Attach the agent to each business’s phone number. Each number carries its own business details, calendar, webhook and caller history, so the businesses stay separate while sharing one set of instructions. Keep per-business values on the number, never on the agent.

Can a single phone number behave differently from the rest?

Yes. A number can have its own instructions, voice, speaking speed, recording setting, keypad setting, calendar and webhook. Anything set on the number wins over the agent; anything left unset follows the agent. See Phone numbers.

Do I have to tell the agent how to handle interruptions?

No. Interruptions, silence, language switching and ending the call are handled by the platform on every call. Your instructions should cover the business side only.

What happens if the caller goes quiet?

The agent asks whether they are still there, and if there is still no answer it says goodbye and hangs up. You choose how long that takes with End on silence, between 5 and 45 seconds.

Can I change the voice for one phone number?

Yes - set voice_id on the number and it overrides the agent’s voice for calls to that number only.

What happens to an agent’s live calls when I change a setting?

Calls already in progress finish on the settings they started with. The change applies to the next call.

Settings available through the API only

These are not in the dashboard. Most agents never need them.

Field	What it does
`stt_confidence`	How sure the speech engine must be before the agent acts on what it heard
`stt_max_silence_ms`	How long a pause may be before the agent treats a sentence as finished
`interruption_sensitivity`	Fine-tunes how easily the caller can interrupt. The dashboard leaves this built in, and the default suits nearly every agent

Fields that are accepted but do nothing

Kept so that existing integrations do not break. Sending them changes nothing.

Field	Use instead
`voice_speed`	`speaking_rate`
`func_send_sms`	`incall_sms_enabled`
`func_cold_transfer`	Your instructions and `transfer_number`
`phone_number`	The phone numbers API

Managing agents


`GET /v1/agents`	List your agents
`GET /v1/agents/:id`	Fetch one
`POST /v1/agents`	Create
`PUT /v1/agents/:id`	Update - send only the fields you are changing
`DELETE /v1/agents/:id`	Delete - calls in progress finish first

GET returns the main agent fields rather than every column. If a setting you saved is missing from the response, it was still saved - send it again with PUT to change it.

Phone numbers - attaching numbers and per-number settings
Controlling the conversation - writing scenario rules
Writing a good agent prompt - the instructions in depth
Agent actions - transfer, text, end call, wait
Create agent - every field, with types and defaults

Get started

Guides

Best practices

Core concepts

In-call actions

Features

Bring your own

SMS

Reference

Agents

One agent, many phone numbers

What you control and what is automatic

You write this

Handled for you

What belongs on the agent and what belongs on the number

Settings

Built in, nothing to set up

An example agent

Can one agent serve different businesses?

Can a single phone number behave differently from the rest?

Do I have to tell the agent how to handle interruptions?

What happens if the caller goes quiet?

Can I change the voice for one phone number?

What happens to an agent’s live calls when I change a setting?

Settings available through the API only

Fields that are accepted but do nothing

Managing agents

​One agent, many phone numbers

​What you control and what is automatic

You write this

Handled for you

​What belongs on the agent and what belongs on the number

​Settings

​Built in, nothing to set up

​An example agent

​Can one agent serve different businesses?

​Can a single phone number behave differently from the rest?

​Do I have to tell the agent how to handle interruptions?

​What happens if the caller goes quiet?

​Can I change the voice for one phone number?

​What happens to an agent’s live calls when I change a setting?

​Settings available through the API only

​Fields that are accepted but do nothing

​Managing agents

​Related

One agent, many phone numbers

What you control and what is automatic

What belongs on the agent and what belongs on the number

Settings

Built in, nothing to set up

An example agent

Can one agent serve different businesses?

Can a single phone number behave differently from the rest?

Do I have to tell the agent how to handle interruptions?

What happens if the caller goes quiet?

Can I change the voice for one phone number?

What happens to an agent’s live calls when I change a setting?

Settings available through the API only

Fields that are accepted but do nothing

Managing agents

Related