SMS is the text messaging system built into every mobile phone sold. No app, no account, no internet connection required. It has been running since 1992 and still reaches more people than any other messaging channel, which is why businesses keep using something that looks primitive next to the alternatives.
This page covers what SMS is, how a message actually travels, where its limits come from, and how it compares with the channels that were supposed to replace it.
What SMS stands for, and what it does
SMS is Short Message Service. It carries plain text between phone numbers over the mobile network’s signalling channel rather than over the internet.
That last detail explains most of SMS’s behaviour. It was designed to travel on spare capacity in the control channel that phones already use to talk to cell towers. It was cheap to add because it used something that already existed, and it inherited that channel’s constraints, including the length limit that still shapes it today.
Where the 160-character limit comes from
A single SMS holds 160 characters. The number is not arbitrary and it is not a business decision. It was the amount of text that fitted into the 140 bytes available in the signalling channel, once encoded at 7 bits a character.
Longer messages still work, because phones split them and reassemble them at the other end. Each part is a segment, and each segment is billed separately, which is why a 200-character message costs twice what a 160-character one does.
Non-Latin characters and emoji change the maths. They need a wider encoding, which drops a segment from 160 characters to 70. One emoji in an otherwise ordinary message can turn one segment into three. If you send at volume, that is the single most expensive detail on this page — our pricing guide works through what it costs.
How a message actually travels
Sending a text sets off a chain most people never see:
- Your phone hands the message to the nearest cell tower over the control channel.
- The tower passes it to your carrier’s SMSC, the short message service centre, which is the piece that actually routes texts.
- The SMSC looks up which network the recipient’s number belongs to and forwards it, either internally or to another carrier.
- The receiving network locates the handset and delivers it.
- If the phone is off or out of range, the SMSC holds the message and retries. This is store-and-forward, and it is why a text sent to a switched-off phone arrives when it comes back on.
Business texting adds a step. Messages sent by software rather than typed by a person are A2P traffic, and in the US they have to be registered with the carriers before they will be delivered at all. Our A2P 10DLC guide covers what that involves.
SMS, MMS and RCS
SMS is plain text, 160 characters a segment, delivered over the cellular network.
MMS carries images, audio and video, and travels over mobile data rather than the signalling channel. It typically costs several times an SMS segment. Ours is billed at three credits.
RCS is the modern successor: branded sender profiles, images, carousels, buttons, read receipts, and end-to-end encryption between iPhone and Android since iOS 26.5. It falls back to SMS when either side does not support it. It is genuinely better and it is still not the default for most business messaging — our RCS guide explains why.
How SMS compares with the alternatives
Against messaging apps. WhatsApp, Messenger and iMessage are richer and free to send, but each only reaches people who use it, on a device that supports it, over a data connection. SMS reaches every mobile number in service.
Against email. Email is cheaper per send and better for length, attachments and anything that needs to be searchable later. It also competes with hundreds of other messages in an inbox with filtering tabs. SMS lands somewhere with almost no competition, which is exactly why it should not be used for routine marketing that could have been an email.
Against push notifications. Push is free and rich, but it requires people to install your app and leave notifications enabled. Most do not. SMS needs neither.
What SMS is genuinely good at
It is worth being specific, because “high open rates” is not a use case.
- Time-critical information. Appointment reminders, delays, closures, on-my-way alerts. Things that lose their value in an hour.
- Reaching people who are not at a computer. Field staff, patients, parents, volunteers, event attendees.
- Anything requiring a reply. Confirmations and short back-and-forth work better over text than over voicemail or email.
- Universal reach. When the list has to include everyone, including people with old phones and no apps.
It is a poor fit for long explanations, anything needing formatting or attachments, and marketing frequent enough to feel intrusive. The channel’s strength is that it interrupts, and that is also the reason to use it sparingly.
Consent is not optional
Because SMS interrupts, it is regulated more tightly than email. In the US the TCPA governs consent, and it is separate from carrier registration: getting a campaign approved tells carriers who you are, it does not give you permission to text anyone.
In practice that means people opt in knowingly, every message identifies who is sending it, STOP works immediately and permanently, and HELP returns something useful. That applies whether you send one message or a million.