SMS deliverability: registration, consent, quiet hours and carrier filtering

Email deliverability is mostly about mailbox providers. In the US, SMS deliverability is up to the carriers, and they act before a text reaches the phone. The CTIA Messaging Principles and Best Practices, the wireless industry's guidelines for business texting, state that service providers may filter or block unwanted messages before they reach consumers. A filtered text doesn't wait in a junk folder. It fails, and the recipient never sees it.

Most of the work happens before your first send.

Register the number before you send

US carriers treat every business text as application-to-person (A2P) traffic and want to know who is sending what. From a phone number, that means one of two registrations:

• 10DLC (an ordinary 10-digit number): you register a brand (your legal entity) and a campaign (the use case, such as marketing or customer care) with The Campaign Registry. TCR's overview says any business texting on 10DLC must register, including businesses that only send non-marketing messages, and puts campaign review at one to four weeks.
• Toll-free numbers: each number goes through toll-free verification, which asks for business details, the use case, sample messages and proof of how people opt in. Since January 31, 2024, texts from unverified toll-free numbers to US and Canadian phones are blocked.

Toll-free review can take a few weeks, and the reviewers set the pace, not your provider. Since September 15, 2026, new toll-free registrations must include public privacy policy and terms URLs.

Most toll-free rejections concern the opt-in or the business details: no explicit SMS consent language, a company name in the opt-in screenshot that doesn't match the registration, a signup that requires agreeing to texts, consent buried in the terms of service, or a website that doesn't load or doesn't match the company.

A registration also fixes what you may send. A toll-free number registered for one use case, such as one-time passwords, can be used only for that and can be revoked if it's used for anything else. A 10DLC campaign works the same way: if you'll send promotions, register a marketing campaign.

Outside the US the rules differ by country. Many countries accept an alphanumeric Sender ID (your brand name in place of a number), and some require registering it with a regulator first. The SMS docs list where each kind of sender works.

Get consent that matches the message

CTIA sets three levels. If the person texts you first, a direct reply needs no further permission. Informational messages (appointment reminders, alerts, order updates) need express consent: the person agrees to texts for that purpose when they give you their number. Promotional messages need express written consent, such as a checked box, a signed form or a keyword they texted. CTIA also notes that adding a call to action like a coupon code to an informational text may make it promotional.

Your call to action, the signup form or keyword ad, should say who will text, what about, from which number, what it may cost and how to stop, with links to your terms and privacy policy. For a recurring program, CTIA asks for one confirmation message before anything else. It should name the program, give a way to get help, say how to opt out, state how often you'll text and mention charges:

Acme Rewards: You're subscribed to sale alerts. Up to 4 msgs/month. Msg & data rates may apply. Reply HELP for help, STOP to cancel.

Keep a record of every opt-in: when, how (web form, keyword, point of sale), what the person saw, the number, and who they are. Carriers and regulators sometimes ask for proof of opt-in, and sending can be paused for a sender who can't produce it. CTIA's principles also say not to send to opt-in lists that were rented, sold or shared, and to remove numbers that show up in carriers' deactivation files, because a reassigned number belongs to someone who never agreed to hear from you.

Honor STOP and answer HELP

Under the FCC's TCPA rules (47 CFR 64.1200(a)(10)), a reply of stop, quit, end, revoke, opt out, cancel or unsubscribe revokes consent, and so does any other reply a reasonable person would read as a request to stop. You must honor it within ten business days. One confirmation text afterward is allowed if it only confirms and carries no marketing. CTIA's guidance matches: one final confirmation, then nothing, and capitalization or punctuation shouldn't decide whether an opt-out counts.

In practice:

• Say how to opt out in the first message, and repeat it from time to time in recurring programs.
• Answer HELP with your business name and a way to reach a person.
• Treat "please stop texting me" the same as STOP.
• Treat a STOP as ending every text that needs that person's consent. Unitpost goes further and opts the number out of its whole SMS channel, service texts included, until the person replies START.
• The FCC revised its revocation rules in an order that takes effect on November 25, 2026. Check the current text of 47 CFR 64.1200 before relying on a narrower reading of what a STOP covers.

On US toll-free numbers, the carrier itself processes STOP and UNSTOP and sends the reply. Record the opt-out on your side as well, or your next campaign will try that number again.

Respect quiet hours

The TCPA rules bar telephone solicitations before 8 a.m. or after 9 p.m. at the recipient's location (47 CFR 64.1200(c)(1)), and paragraph (e) of the same section applies that rule to texts sent to wireless numbers. The clock that counts is the recipient's. An area code shows where a number was issued, which isn't always where its owner lives now. State laws and other countries can be stricter, so check the places you send to.

Inside the allowed hours, timing still matters: texts at dinner time or late at night push people to unsubscribe. Messages the person is waiting for, like a login code or a delivery alert, are different: send those immediately.

How carriers filter

Carriers audit bulk senders, and throttle or block messages from senders they find sending unsolicited messages. The patterns that draw filtering are well documented:

• Traffic that doesn't match the registration, such as promotions sent over a number registered for alerts.
• Snowshoeing, which CTIA defines as spreading messages across many sending numbers, usually to get around per-number limits.
• Public link shorteners. CTIA wants shortened links on a domain dedicated to you, and carriers tend to filter links on free shortening services.
• Texts that look like phishing: no business name, a message dressed up as a personal text from "Jane", or a money offer that sounds too good to be true.
• Sending to people who never opted in, which is exactly the unsolicited traffic carriers block.

Filtering rarely announces itself. What you see is failed messages with carrier error codes, so check failures by error after every campaign.

Content that gets blocked

Some categories are filtered or refused regardless of consent. Carriers' restricted content includes a group the industry labels S.H.A.F.T. (sex, hate, alcohol, firearms, tobacco and vaping), plus gambling, high-risk loans and debt relief, get-rich-quick offers, cannabis and CBD, prescription drugs, phishing and third-party lead generation, all heavily filtered or blocked outright (Unitpost's list is in the SMS Messaging Policy). CTIA's principles list content that doesn't meet age-gating requirements among the traffic senders should prevent, so age-restricted products need an age gate and a registration that declares them, where carriers allow them at all.

Within allowed categories, a few writing habits keep texts clean:

• Start with your brand name so the recipient knows who is texting.
• Link to your own domain, and make sure the page identifies you.
• Write offers plainly: no currency symbols as decoration ("Save big $$$") and no heavy abbreviations like gr8 and 2day.
• Watch the character set. One character outside the GSM alphabet, such as ® or an emoji, switches the message to an encoding with 70 characters per part instead of 160, so a short text can split into several billed parts.