A2P 10DLC.In plain English.

If you send texts to US numbers from standard long-code numbers, you need to register. You register a brand and then each texting use case, called a campaign, through your messaging provider, and The Campaign Registry verifies brands and providers before they can send.

Campaign reviews take up to 5 business days for standard use cases, according to Twilio. Twilio blocks messages to US numbers from a 10DLC number that isn't linked to an approved campaign.

Every registration needs a privacy policy and terms and conditions, and Twilio says verbal consent is not enough for marketing use cases: you need express written consent, such as a website checkbox.

If a customer texts you first and you simply reply, the industry's messaging guidelines treat that as a conversation that needs no further permission.

Example: a dental office registers its brand and a 'customer care' campaign before turning on appointment reminders, and adds an unticked consent checkbox and SMS terms to its booking form.