What is a concatenated SMS?
A concatenated (or multipart) SMS is a message longer than a single segment — 160 GSM-7 characters, or 70 in UCS-2 — split across multiple SMS parts and reassembled into one message on the handset. A User Data Header (UDH) in each part carries a reference number and sequence so the phone stitches them in order. Concatenation reduces the per-segment payload to 153/67 characters to make room for the UDH, and each part is billed separately. See /answers/what-is-udh-sms, /answers/what-is-encoding-gsm-7-ucs-2, and /tools/sms-segment-calculator.
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