The [SEND_SMS:] tag sends a text message to the caller from your Twilio number. Useful for booking confirmations, links, addresses, payment links, or anything easier to read than hear.
Syntax
[SEND_SMS: Your text message goes here]
Everything between the colon and the closing bracket is the SMS body. The agent continues the conversation naturally — the SMS is sent in the background.
Example
Caller: “Can you send me the address?”
Agent: “Sure, sending it now. [SEND_SMS: Acme Dental, 12 High Street, Croydon CR0 1AB. See you Tuesday at 2pm.]”
When to use
Tell the agent in its system_prompt when to text the caller. Example snippet from a system_prompt:
ACTIONS
- Send the address by SMS if asked where you are: [SEND_SMS: 12 High Street, Croydon CR0 1AB]
- After booking, text confirmation: [SEND_SMS: Your appointment with Acme Dental is confirmed.]
- Send the payment link when collecting deposits: [SEND_SMS: Pay your deposit here: https://pay.example.com/abc]
Post-call SMS
Separate from the mid-call tag, Nixflex can automatically send a text after a call ends. There are two ways to set it up, depending on your structure.
Agent-level template
Set a post_call_sms_template on the agent (with post_call_sms_enabled: true) via the Update agent endpoint. It then fires automatically after every answered call on that agent - both inbound and outbound.
curl -X PUT https://api.nixflex.com/v1/agents/agent_125207e452f8714a \
-H "Authorization: Bearer KEY_ID:KEY_SECRET" \
-H "Content-Type: application/json" \
-d '{ "post_call_sms_enabled": true, "post_call_sms_template": "Thanks for speaking with us today, {caller_name}. We will be in touch." }'
| Variable | Value |
|---|
{caller_name} | The caller’s name, if captured during the call |
{business_name} | The agent’s name |
{call_summary} | A short summary of the call |
{agent_name} | The agent’s name |
Write the template so it reads naturally for both inbound and outbound calls if the agent handles both. “Thanks for calling” only fits inbound - prefer neutral wording like “Thanks for speaking with us today”, or use variables to keep it flexible.
Your own, from the webhook
If one agent serves many separate businesses, a single agent-level template would send the same text to every business’s customers. In that case, send the post-call SMS yourself: receive the webhook after each call and send a message tailored to that business from its own number, using the Send SMS API. Each business gets its own wording, branding, and number, with full control over when a message goes out.
When it sends
Post-call SMS sends once, shortly after the call ends, from the business’s own Twilio number. It is skipped when the call reached voicemail, when no template is configured, or when the caller’s number is not available - so messages only go out when they make sense.
Inbound SMS replies
When a caller replies to a Nixflex SMS, the agent responds automatically using its system_prompt. The agent’s regular instructions handle the reply — there is no separate reply-handling system.
Key principle: if you send SMS that customers might reply to, include the relevant context in the agent’s system_prompt so the agent knows how to handle replies, questions, or follow-up calls about that SMS.
Sending SMS via the API
Outside of a live call, send SMS programmatically using the message field. See Send SMS API.
Bulk SMS campaigns
For sending the same message to many numbers, use SMS campaigns.
Compliance
UK only at launch. US SMS requires A2P 10DLC registration on your Twilio account. Until that is complete, US-bound messages are rejected by carriers.You are responsible for SMS compliance: opt-in records, opt-out handling (STOP/HELP keywords), and timing restrictions. Nixflex provides the infrastructure; the compliance obligation stays with you.
Twilio billing
SMS is sent via your Twilio account (the same one attached to the phone number). Twilio charges per message at standard rates. Nixflex does not mark up SMS pricing. 
