What happens when you send a text message?
It travels through the signalling channel of the mobile network rather than as ordinary data — a design accident from the 1980s that explains almost everything odd about SMS.
The origin. SMS was devised to use spare capacity in the control channel that phones and masts already used to coordinate calls and check signal. That channel had a small fixed payload available, which is why a message is limited to 160 characters in the basic encoding — the number was chosen because engineers judged it sufficient for typical messages, not for any deeper reason.
The path a message takes: your phone sends it to the nearest mast; it reaches a short message service centre, which is the crucial component; the centre looks up where the recipient currently is; and it delivers the message, or stores it and retries if the phone is off or out of coverage. SMS is a store-and-forward system, which is why messages arrive in a burst when a phone comes back into signal, and why they can arrive out of order.
What this explains:
Why long messages split. Over 160 characters, the message is broken into concatenated parts with a header, reducing each part's payload — so a long message costs more parts than the arithmetic suggests.
Why one emoji shortens your message dramatically. A non-Latin character forces a switch to a 16-bit encoding, cutting the limit to 70 characters per part.
Why delivery reports are unreliable. They confirm handoff, not reading.
Why SMS still works when data does not — it uses a different channel entirely, which is why it remains the fallback for emergency alerts and authentication codes.
Why SMS is a poor second factor. It is not encrypted, it is vulnerable to SIM swap attacks and to interception through signalling network weaknesses, and messages pass through intermediaries. Security guidance increasingly discourages it where alternatives exist.
RCS is the modern replacement, carrying media, read receipts and — increasingly — encryption over data rather than the signalling channel.