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SMS is far more regulated than WhatsApp. Two things are mandatory: honouring opt-outs and registering your traffic before you send.

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Keywords

Lumina recognises three keyword groups before routing and before the model sees the message. They aren’t configurable, and they work even with no agent on duty.
The keywords are English only. There’s no Spanish list: a customer texting “BAJA” or “PARAR” does not trigger the automatic opt-out.
Conversations thread with the Resolved pill where the customer texted STOP, followed by the confirmation note You have been unsubscribed from messages from this number and the footer This session has ended

STOP: the confirmation goes out and the session resolves

If the contact texts again later, a new session opens, but the agent’s reply is blocked:
A new SMS session in AI handling where the customer's message is followed by the note Message not sent: this contact has opted out of SMS messages from this number

After STOP, every reply is blocked and the thread says so

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Matching is exact

The keyword has to be the entire message — trimmed, case-insensitive — not part of it. “Please stop texting me” does not trigger it.
That’s deliberate: if containing the word were enough, a message like “stop by the shop at 5” would silently mute the customer, with no way for them to notice or undo it.

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The AI cannot opt someone out

Lumina has no opt-out tool. A customer writing “please stop texting me” or “no me escribas más” gets an acknowledgement at best — the contact is not suppressed.Handle it in the prompt: “If they ask to stop receiving messages, tell them to reply STOP, then end the conversation” — and attach an End Chat tool so it can. Don’t tell the model to “perform the opt-out”: it can’t.

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Suppression is per contact, account-wide

Not per conversation: once opted out, that contact is suppressed on the SMS channel across every number and every agent on the account.
  • Starting a chat with a suppressed contact is refused: “This contact has opted out of SMS messages. Release them from the suppression list to message them again.”
  • A blocked send inside an open thread leaves the note “Message not sent: this contact has opted out of SMS messages from this number.”
  • The Send SMS tool refuses opted-out recipients.
Opt-out confirmations are still sent to suppressed contacts — that’s what regulation requires.
There is no screen to view or release the suppression list. To release a contact, contact Dialtu support — or the customer texts START.

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The carrier keeps its own list

A carrier-level opt-out blocks sends at the network even if Dialtu still shows the contact as active. To receive messages again, the customer has to text START to the carrier, not just to you.

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A2P 10DLC registration

In the United States, all business-to-person traffic on local numbers must be registered. It isn’t handled inside Dialtu: the Dialtu team helps you through it.
Start this before anything else. Approval takes days to weeks. It’s the critical path of any SMS project.
Things worth knowing before you submit:
This is the most common rejection reason. The legal name and number must match what the tax authority holds, character for character — a trading name won’t do.
Register it as conversational / customer care, not as marketing.
Misregistering the use case is grounds for suspension.
If you leave the carrier’s automatic opt-out handling switched on, the customer receives two confirmations: the carrier’s and Dialtu’s.
Once the carrier records the opt-out, sends fail at the network level even if Dialtu still shows the contact as active. To receive messages again, the customer has to text START to the carrier, not just to you.

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Good practice

Identify yourself in the first message. Who you are and why you’re writing.
Tell people how to stop. Put “Reply STOP to opt out” in the first message of any conversation you start.
Respect the hours. Use time-based routing so an inbound agent isn’t answering at 3am.
Mind the hours on outbound. Outbound isn’t gated by the schedule — use a Salesflow Wait or a scheduled chat.
Be brief. Long messages are split, and each part is billed.

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Next step

What SMS costs

What all of this costs.