SMS API for developers and AI agents

What the SMS API does

Use SMS for a short message, such as an appointment reminder or a delivery update. Your application receives the reply in the same workflow.

• Send SMS: use a messaging-capable number configured for your destination and use case.
• Receive replies: subscribe to signed webhooks so your application can react to incoming messages.
• Track delivery: inspect message status before deciding whether to follow up.
• Honor opt-outs: Hail blocks sends to recipients who have opted out until they opt back in.

Start with your own phone and a sender that is ready to send. An API key alone does not make every number eligible for messaging in every country.

See what a phone number costs in each country: Number prices by country.

Check message length before you send

A single text in your editor can become several billable SMS segments. Encoding and message length affect the count, so test the actual copy, including punctuation and emoji, in the SMS length calculator.

Keep essential information near the beginning. If the update needs several paragraphs, send a short notice and put the details in an email. Check the rate card before estimating the cost of a campaign.

How long does 10DLC registration take?

For application-to-person messaging over US local ten-digit numbers, plan for brand registration, campaign review, and number setup. These are separate steps; approval of the brand does not mean the number is ready to send.

As one provider example, AWS lists 1–2 business days for US brand registration and up to four weeks for campaign registration. International brand registration can take longer. These are AWS estimates for individual steps. Hail does not promise these times.

Check the current AWS 10DLC registration table and the status of your actual sender before setting a launch date.

Prepare the application before submitting it

• Make your business name, registration details, and website agree.
• Document how recipients opt in, with a public page that reviewers can inspect.
• Provide sample messages that represent what you will actually send.
• Describe opt-out and help handling, and resolve the stated rejection reason before resubmitting.

10DLC is specific to US local-number messaging. Other number types and destinations have their own requirements. Do not send the same traffic from a different number while a registration is pending.

Use SMS alongside calls and email

Send a written confirmation after a voice-agent call, or let a reply trigger a task in your backend. Your application chooses the next action. Hail provides the channels and events. Read the API reference or connect your MCP client.