Flash SMS is a text message sent as "Class 0," which tells the phone to show it on screen right away instead of saving it to the messages inbox. Once the recipient dismisses it, the message is usually gone, unless the phone offers a "save" option and they tap it.
The idea sounds useful for urgent alerts. In practice, US businesses almost never send flash SMS. Most US routes don't carry it reliably, iPhones don't display it as a pop-up, and recipients who do see one tend to read it as spam. This post covers how Class 0 works, where it is supported, why it backfires, and what to send instead when a message is time-sensitive. We use Ready, which we make, for the worked examples.
How Class 0 messages work
Every SMS carries a small header field called the Data Coding Scheme. Two things live in that field: the character set (GSM-7 or unicode) and, optionally, a message class. The GSM standard defines four classes:
| Class | What the phone is told to do | Common name |
|---|---|---|
| 0 | Display immediately, don't store by default | Flash SMS |
| 1 | Store on the device | Normal text (the default in practice) |
| 2 | Store on the SIM card | SIM-specific (used for carrier provisioning) |
| 3 | Pass to connected equipment | Rarely used |
A Class 0 message does not land in the conversation thread. It appears as an overlay or dialog on top of whatever the person is doing. There's usually no thread history, often no reply button, and once it's closed there's nothing to scroll back to.
Flash SMS is not "silent SMS" or an emergency alert
Two things often get mixed up with Class 0:
- Silent SMS (Type 0) uses a different header field. The phone acknowledges it and shows nothing at all. Networks use it for diagnostics, and it has nothing to do with marketing or alerts.
- Wireless Emergency Alerts (the Amber Alert or extreme-weather pop-ups) look a lot like flash messages, but they aren't SMS. They use cell broadcast, which pushes one message to every phone on a set of towers. Only authorized government agencies can send them. No business texting product can send one.
When people say they want "a text that pops up like an emergency alert," what they're really describing is cell broadcast. Businesses can't use it.
Flash SMS support in the US: devices and carriers
Two things have to cooperate for flash SMS to work: the route the message travels and the phone that receives it.
The route
US business texting runs over A2P 10DLC (registered 10-digit numbers), toll-free numbers or short codes. The message passes through your provider, an aggregator and then the carrier. Each hop can normalize or drop header fields it doesn't expect. Most US A2P routes don't let you set the message class at all. Those that accept the setting don't promise it reaches the handset intact. A message you submit as Class 0 may arrive as an ordinary text, or it may not arrive.
Ready's messaging API has no class or flash setting. Every text it sends is a standard message that lands in the recipient's thread. That's a deliberate choice, and the reasons are below.
The phone
- Android: many Android phones have historically shown Class 0 messages as a pop-up dialog, often with a save option. Behavior differs by manufacturer, OS version and default messaging app.
- iPhone: iPhones don't show Class 0 as a pop-up. Apple doesn't document how iOS handles the class, so you can't count on any particular result.
Roughly half of US smartphones are iPhones. On those phones, even a perfect Class 0 delivery wouldn't produce the flash behavior you were paying for. You can't design an alert around a feature that works on some Android phones and on no iPhones.
Why flash messages look like spam
Suppose the route and the device both cooperate. The recipient still sees something odd: a box appears over their screen, from a number, with no conversation history and often no way to reply. That's also what many phishing and "your account is locked" scams look like. Flash messages built that reputation outside the US, and anyone who has seen one tends to dismiss it on sight.
There are three practical problems beyond the first impression:
- No record. The message is gone once dismissed. If it carried a confirmation code, an address change or an appointment time, the customer can't find it again. Then they call you, or they miss the appointment.
- No easy opt-out. Opt-out handling depends on the recipient replying STOP in a thread. A pop-up with no thread makes that harder. That's a bad position for a sender, because consent and opt-out handling are the sender's responsibility.
- Carrier filtering. Carriers filter on content and on patterns that look abusive. Unusual header behavior from a business number is a pattern you don't want to show. Registration doesn't protect you from content-level filtering, as we explain in why approved campaigns still get filtered.
What to send instead for urgent alerts
What makes an alert work is speed, a clear message and a sender the customer recognizes. Each of the options below covers those three things better than Class 0.
1. A standard SMS written to be read in two seconds
Put the urgent part in the first few words, because that's what shows in the lock-screen preview:
Acme Plumbing: Your 2 PM visit moved to 4 PM today. Reply 1 to confirm, 2 to reschedule. Reply STOP to opt out.
That's 116 characters with no emoji, so it's one GSM-7 segment. It lands in the thread, the customer can reply, and it's still there tomorrow.
Register the campaign under the use case that matches what you actually send. Alerts and reminders filed as marketing get throttled. The use-case picking guide covers how to choose.
2. A blue line for customers you already text
Ready's blue line sends as iMessage, so the text shows up blue on iPhones with your business name and photo. That makes the sender easy to recognize, which is the part flash SMS gets wrong. It costs $99 a month per line, plus 6 cents a segment for each text sent. It's unlimited with people you already talk to, but each line can start only 50 new conversations a day. Campaigns send from regular numbers, not the blue line. It works for one-to-one urgent messages to existing customers, not for alerting a whole list. See the blue line guide and our post on blue bubble texting for business.
3. A phone call for anything truly critical
If the message can't wait and can't be missed, call. The Ready Dialer handles click-to-call from the dashboard or GoHighLevel at $0.025/min, and each started minute is billed as a full minute. It's manual calling with no auto-dial. A practical pattern is to send the text first, then call the people who haven't replied.
4. Push notifications, if you have an app
If your customers already have your app installed, a push notification is the closest thing to a pop-up a business can legitimately send. It doesn't help customers who don't have the app.
Worked example: an outage alert to 2,000 customers
Assumptions: a service business needs to tell 2,000 opted-in customers about a same-day outage. The account is on Ready Standard with a registered 10DLC campaign. Local presence, scrubbing and validation are off.
Message: 150 characters, plain GSM-7 text, no emoji, which is 1 segment.
Cost: 2,000 × 1 × $0.0075 = $15 plus 2,000 × $0.0045 in carrier fees.
Now add one emoji to the same 150 characters. The whole message switches to unicode, where the limit is 70 characters for a single segment and 67 per segment once it splits. 150 ÷ 67 rounds up to 3 segments. The cost is now 2,000 × 3 × $0.0075 = $45 plus 6,000 × $0.0045. That's 3x the bill for one character.
Ready's all-in prices per segment include the carrier fee but not screening: Scrub lists and Number validation, the two switches in Settings under Compliance, each add $0.003 per segment when switched on, shown as screening on your bill, and both are off by default. Unless you turn the opt-out footer off, the first text to each contact, and one again from time to time, also gets "Reply STOP to opt out" and your business name if the message does not already have them, so a text close to the segment limit can bill one more segment.
Speed: carriers meter segments per second, not messages. At an assumed 3 segments per second (typical for a fresh standard campaign), 2,000 segments take about 11 minutes to go out. The 6,000-segment emoji version takes about 33 minutes. At an assumed 10 segments per second, the plain version finishes in under 4 minutes.
For urgent alerts, then, segment count and throughput determine how fast the message arrives. Message class doesn't. The cheapest way to speed up an alert is to keep it to one GSM-7 segment. Higher daily caps come from trust score. Brand vetting ($79 Standard / $179 Enhanced, one-time) can raise the score, but it doesn't change the per-second rate.
One tradeoff to know: Ready enforces quiet hours based on the recipient's area. Campaign and drip texts that would land in their quiet hours are held until the window opens. An inbox or API text is held only if sending reopens within 6 hours; otherwise it is refused with the reopen time and you send it again. A held text is charged only when it sends. That protects you from TCPA exposure. It also means a 2 AM emergency should be a phone call, not a text.
To send this through the API, POST /v1/messages/bulk takes up to 100 recipients per call, so 2,000 contacts is 20 batches. Send an Idempotency-Key with each batch so a retry never sends twice. Opt-outs and known landlines are refused up front and not charged. Details are in the bulk messages docs and the messages docs.
The practical takeaway
Flash SMS solves a problem US businesses don't really have, and it creates three they do: inconsistent delivery, no record for the customer and a spam-like first impression. For urgent messages:
- Send a standard text with the urgent detail in the first few words, kept to one segment.
- Register the campaign as what it is, not as marketing.
- Use a blue line when recognition matters with existing customers.
- Call when the message can't be missed.
To see how a one-segment alert reads on a real phone, the first 20 texts on Ready are free one-to-one sends. You'll need a card on file. Sign up and text yourself before you register.
FAQ
What is flash SMS?
Flash SMS is a Class 0 text message. The phone displays it immediately on screen instead of saving it to the inbox, and it usually disappears once dismissed unless the user saves it.
What is a Class 0 SMS?
Class 0 is one of four message classes in the GSM standard, set in the message's Data Coding Scheme field. Class 0 means "display immediately, don't store by default." Normal texts behave like Class 1 and are stored on the device.
Do iPhones support flash SMS?
iPhones don't display Class 0 messages as a pop-up, and Apple doesn't document how iOS handles them. Some Android phones do show the pop-up, but behavior varies by maker and messaging app.
Can I send flash SMS through Ready?
No. Ready's messaging API has no flash or message-class setting. Every text is a standard message that lands in the recipient's thread, where they can reply, opt out and find it later.
Are emergency alerts on my phone flash SMS?
No. Wireless Emergency Alerts use cell broadcast, not SMS, and only authorized government agencies can send them. Businesses can't send them through any texting platform.