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This diagram shows how an SMS message travels from sender to recipient — through the carrier network, SMSC store-and-forward system, and back.
160 characters
A single SMS is limited to 160 7-bit characters. Longer messages are split into multiple parts and reassembled by the receiving phone.
Store and forward
If the recipient is offline, the SMSC stores your message for up to 48 hours (configurable) and retries delivery when they reconnect.
Delivery reports
When the SMSC delivers your message to the destination, it sends a delivery receipt back to you — that's the ✓✓ tick you see.
SS7 signalling
SMS travels over SS7 — the same signalling network as voice calls. This is why SMS works even with weak signal when calls don't.
No encryption
Traditional SMS is unencrypted between carriers. Your carrier can read every SMS. Use Signal or WhatsApp for end-to-end encryption.
A2P SMS
Application-to-Person SMS (OTPs, alerts) goes through aggregators like Twilio who connect to carriers via SMPP protocol.