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.
Send one message
curl -X POST https://api.nixflex.com/v1/sms \
-H "Authorization: Bearer KEY_ID:KEY_SECRET" \
-H "Content-Type: application/json" \
-d '{
"agent_id": "agent_125207e452f8714a",
"to": "+447386172392",
"from_number": "+447446466847",
"message": "Reminder: your appointment is tomorrow at 2pm. Reply YES to confirm or NO to reschedule."
}'
| Field | Required | Notes |
|---|
agent_id | Yes | The agent this SMS belongs to (determines reply behaviour) |
to | Yes | Recipient in E.164 format |
from_number | Yes | One of your imported Twilio numbers |
message | Yes | The message text. 600 chars recommended for reliable delivery. Twilio’s hard cap is 1600, but UK carriers often reject longer messages. See SMS length guide below. |
{
"message_id": "SM1234567890",
"status": "queued",
"to": "+447386172392",
"from_number": "+447446466847"
}
delivered, failed, etc.) reflects in the message record within ~30 seconds.
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.
| Length | Segments | Cost | Reliability |
|---|
| 160 chars | 1 | Cheapest | 99%+ delivery |
| 320 chars | 2 | 2× | 99%+ delivery |
| 480 chars | 3 | 3× | 95-99% delivery |
| 600 chars | 4 | 4× | ~95% delivery |
| 1000+ chars | 7+ | High | Often rejected by UK carriers |
- 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.
How replies are handled
When someone replies to your SMS, Twilio routes the inbound message to the agent that owns the from_number. The agent answers using its system_prompt — the same prompt that drives phone calls.
This means: if you expect customers might reply with questions about what you sent, mention the context in the agent’s system_prompt before you send the SMS.
For example, if you’re sending appointment reminders, add this to the agent system_prompt:
You are the receptionist at Acme Dental. If a customer replies to an appointment reminder with YES, confirm the booking. If they reply NO or want to reschedule, offer alternative slots. If they ask any question about their appointment, answer warmly using the information you have.
Now whether the customer replies to the SMS, sends a fresh text later, or calls in — the agent is ready.
Tracking
Every SMS gets a message_id you can use to fetch status:
curl https://api.nixflex.com/v1/sms/SM1234567890 \
-H "Authorization: Bearer KEY_ID:KEY_SECRET"
status (queued, sent, delivered, failed, undelivered), error code if any, and the conversation thread if replies happened.
Twilio 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.
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.
Bulk campaigns
For sending the same message to many recipients in one call, use SMS campaigns.
Compliance
UK only at launch. US SMS requires A2P 10DLC registration on your Twilio account. 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. Pricing
SMS uses your Twilio account directly. Twilio charges per message at 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. 
