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Beyond mid-call and post-call SMS, you can send standalone messages via the API. Useful for appointment reminders, status updates, follow-ups — anything that doesn’t need a phone call.

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Send one message

Response:
Status updates flow through your carrier’s status callbacks. The final status (delivered, failed, etc.) reflects in the message record within ~30 seconds.

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SMS length and deliverability

Even though Twilio’s API accepts messages up to 1600 characters, real-world delivery is more nuanced. Carriers — especially UK mobile carriers — silently reject long messages without giving an error code. Recommended ceiling: 600 characters. This stays within reliable delivery limits while leaving room for variable expansion in templates. Each segment costs ~£0.04 to send. A 600-char message costs ~£0.16 per send, versus £0.04 for 160 chars. If you need longer content, consider:
  • Splitting into multiple SMS
  • Including a link to a webpage
  • Switching to email for that customer
For Post-Call SMS templates configured in the dashboard, the textarea is hard-capped at 600 chars to enforce the safe zone.

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How replies are handled

When someone replies, your carrier routes the message to Nixflex and the SMS agent on that number answers it.
Replies use the number’s sms_prompt, not the agent’s system_prompt. SMS is an independent channel: the call prompt and the number’s custom_prompt are deliberately not used. Put your reply instructions in the call prompt and the SMS agent will never see them - it falls back to a short default that knows nothing about your business.
So if you expect replies, set the number’s sms_prompt before you send:
Auto-reply is off by default. A number only answers texts once sms_reply_enabled is true. Without it, replies arrive and are stored but nothing is sent back. See SMS agent for the full reply behaviour, including booking over text.

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Carrier support

Sending works on both carriers. Nixflex applies no geographic limit of its own - coverage is whatever your own Twilio or Telnyx account permits, and if your carrier refuses a destination the error tells you which setting to change.
Telnyx: delivery status is not tracked yet. A message sent from a Telnyx number goes out normally, but its status stays at sent and never moves to delivered or failed. Use a Twilio number if you need delivery confirmation. Everything else - sending, receiving, auto-reply - works on both.

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Tracking

Every SMS gets a message_id you can use to fetch status:
Response includes status (queued, sent, delivered, failed, undelivered), error code if any, and the conversation thread if replies happened.
Status meanings: delivered means the carrier confirmed delivery to the handset. sent means Twilio handed off to the carrier but never got confirmation — we treat this as a soft failure in the dashboard. failed and undelivered are hard failures.

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Mid-call and post-call SMS

For sending SMS from inside a live call (booking confirmations, links) or automatically after every call (summaries), see Send SMS action. Those use the [SEND_SMS:] tag and post_call_sms_template on the agent — no separate API call needed.

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Bulk campaigns

For sending the same message to many recipients in one call, use SMS campaigns.

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Compliance

UK only at launch. US SMS requires A2P 10DLC registration on your carrier account - this is a US carrier requirement, not a Nixflex one. Without it, US messages are rejected by carriers.You’re responsible for:
  • Opt-in records (you have permission to text these numbers)
  • Handling STOP / UNSUBSCRIBE keywords (Twilio handles this automatically by default)
  • Respecting time-of-day rules (no messages 9pm-9am in the recipient’s timezone)
Nixflex provides infrastructure; compliance stays with you.

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Pricing

SMS goes through your own carrier account. Your carrier charges per message at their standard rates:
  • UK domestic: ~£0.04 per message
  • US (after A2P): ~$0.0075 per segment
  • International: varies by country
Nixflex doesn’t mark up SMS pricing.