What is SMS? How text messaging works, character limits, pricing and OTP use (Hong Kong & Taiwan)

Key takeaways

• SMS is the text-messaging service built into the mobile network itself: no app, no data connection, only a signal is needed.
• One SMS holds 160 Latin characters or 70 Chinese characters; anything longer is split into segments, each billed separately.
• Taiwan calls it 簡訊 and Hong Kong calls it 短訊; both mean the same service.
• One-time passcodes (OTP) still go by SMS first because it reaches every handset without an app or prior opt-in to a platform.
• Hong Kong registers business sender names with OFCA (the ”#” prefix); Taiwan requires commercial SMS containing a URL or phone number to be white-listed first.

People searching “what is SMS” have usually just received one, are setting up phone verification, or are about to send messages to customers for the first time. This article covers three things: how SMS works, what one message costs and holds, and what a business needs to know before sending in Hong Kong and Taiwan.

First, the words: 簡訊, 短訊 and SMS are the same thing

SMS stands for Short Message Service. Chinese has no single translation:

Region	Usual term	Related vocabulary
Taiwan	簡訊	簡訊驗證碼 (SMS verification code), 企業簡訊 (business SMS), 電信業者 (carrier), 門號 (mobile number)
Hong Kong	短訊	短訊驗證碼, 企業短訊, 電訊商, 手機號碼
Mainland China	短信	验证码短信, 运营商

Whichever term you meet, it is the same service.

How SMS works

SMS arrived in the 1990s as part of the GSM mobile standard. A message does not travel over the internet; it goes through the carrier’s signalling channel, is held by a short message service centre (SMSC) inside the network, and is handed to the recipient’s handset. That gives SMS three properties:

1. No app required — every phone can send and receive it, including feature phones.
2. No data required — mobile data off, no Wi-Fi, still delivered as long as there is a signal.
3. No prior contact required — knowing the number is enough; there is no “add me first” step as with messaging apps.

Those three points are why banks, brokerages and e-commerce platforms still send verification codes and transaction alerts by SMS: it is the only channel that reaches every mobile user.

How many characters fit in one SMS

One SMS carries a fixed 140 bytes. How many characters that is depends on the encoding:

Content	Encoding	Single message	Per segment when split
Latin letters and digits only	GSM-7	160 characters	153 characters
Any Chinese character, emoji or full-width punctuation	UCS-2	70 characters	67 characters

A single Chinese character switches the whole message to UCS-2 and drops the limit from 160 to 70. Longer messages are split into segments that the handset reassembles, so the reader sees one long message, but every segment is billed on its own. The full rules for Chinese text and segmentation are in How Hong Kong SMS billing works — characters, Unicode and segmentation.

How SMS is charged

For consumers, SMS is normally bundled into the monthly plan. Bulk sending by businesses works differently:

• Per message — each segment counts as one message, priced by destination country and volume.
• On submission — billing usually happens when the message is accepted by the carrier; whether the recipient opens it makes no difference.
• Failures not billed — a malformed number or rejected content that fails before submission is normally not charged; the exact rule depends on the provider.

When choosing a provider, what matters more than the price is whether the delivery reports are real. An API “success” only means the platform accepted the request; whether the message reached the handset is what the carrier’s delivery status says. See SMS DLR status codes explained — what SMD, DELIVRD, UNDELIV and REJECTD actually mean.

SMS versus MMS

MMS (Multimedia Messaging Service) extends SMS with images, video and longer text. The biggest difference is the route: SMS travels over the signalling channel, MMS content is downloaded over a data connection. With mobile data switched off an SMS still arrives, while an MMS may show only a download prompt. For promotions, use MMS for image-led content and SMS for anything that must arrive. See What Is MMS? MMS Messages, Multimedia SMS, and How They Differ from SMS (Hong Kong Business Guide).

Why one-time passcodes still use SMS

The phone verification code (OTP, one-time passcode) is the most common business use of SMS, for straightforward reasons:

• Widest reach — the recipient needs no app and no account on any platform.
• Tied to the SIM — the code goes to the number on the SIM, which confirms that the number is in the user’s hands.
• Immediate — normally delivered within seconds.

SMS OTP can be abused, though: attackers trigger your send-code endpoint at scale to burn your SMS budget. Code validity, resend limits and rate limits all need designing. See What is an SMS code? How one-time passcodes (OTP) work.

Sending as a business: Hong Kong and Taiwan differ

Both places have anti-fraud measures, but they pull in opposite directions. Hong Kong regulates who is sending; Taiwan regulates what is in the message.

	Hong Kong	Taiwan
Sender display	Alphanumeric sender names (e.g. a company name) are supported; names registered with OFCA show a ”#” prefix	Alphanumeric sender names are not supported; business SMS arrives from a numeric number
Fixed identity	The registered ”#” name is the trust mark	A 6-digit dedicated short code can be applied for, one code per company
Content rules	Promotional messages fall under the Unsolicited Electronic Messages Ordinance and must carry an unsubscribe facility	Commercial SMS containing a URL or phone number must be white-listed first; plain text is unaffected
Opt-out	Do-not-call Registers	Personal Data Protection Act Article 20: the first marketing contact must offer a way to refuse, and marketing must stop once refused

Hong Kong: the ”#” sender registration

Hong Kong’s Office of the Communications Authority (OFCA) runs the SMS Sender Registration Scheme: sender names of registered organisations are displayed with a ”#” prefix so recipients can tell genuine messages apart. The application steps are in Registering a # Sender ID in Hong Kong — a practical guide to the OFCA scheme, and the rules for promotional messages in Hong Kong promotional messaging compliance checklist — what the UEMO requires of senders.

Taiwan: white-list registration

Since 18 November 2024, Taiwan’s NCC has required carriers to tighten controls on commercial SMS: if a message contains a URL, a short URL or a phone number, the carrier checks before sending whether the sending business has registered the matching details, and messages that do not match, or come from an unregistered sender, are not sent. Text-only messages are unaffected, so plain-text verification codes and transaction alerts continue as before.

A business that needs links or phone numbers in its messages should note:

• Where to register — through a carrier or an SMS relay provider.
• What is registered — the company’s identity, the web domains the messages will use, and the phone numbers. The exact documents depend on the accepting provider; company registration details are the norm.
• The domain must match — carriers check that the sending company and the registered domain correspond, so a third-party short-URL service will not match; use your own domain.
• Changes need a new application — a new domain or phone number means registering again.
• White-listing is not permanent — a registered sender whose messages are reported as fraudulent is re-examined.

For personal numbers, NCC’s guidance is a limit of 50 messages a day; beyond that, messages are held until the user passes further identity verification.

How a business gets started

In both Hong Kong and Taiwan, bulk SMS goes through a sending platform or API, not through a handset. The usual path is:

1. Open an account and set up a sender name (Hong Kong) or understand how the number will be displayed (Taiwan).
2. Add a few of your own numbers as test recipients and send a plain-text message to see delivery time and how it appears.
3. Once delivery reports look right, integrate the API or import your list.

UFOSEND’s platform supports Hong Kong and Taiwan numbers; create an account and send test messages to your own numbers before deciding whether to go live.

Frequently asked questions

How is SMS different from iMessage or WhatsApp? SMS travels over the carrier network and the recipient needs nothing installed. iMessage and WhatsApp travel over the internet and both sides need the app and a data connection.

Can I receive SMS without data or Wi-Fi? Yes. SMS only needs a mobile signal.

How many Chinese characters fit in one SMS? 70. Longer messages are split into segments of 67 characters, each billed separately.

Is SMS free? For consumers it is usually included in the monthly plan. Businesses pay per message, priced by destination and volume.

Does a Taiwan SMS verification code need white-listing? No. A plain-text code does not. The white-list applies only to commercial messages containing a URL, short URL or phone number.

Can a Hong Kong company use its own name as the sender? Yes, and once registered with OFCA it is shown with a ”#” prefix. Taiwan does not support alphanumeric sender names; messages there show a number.