UCS-2
UCS-2 is the 16-bit encoding triggered as soon as an SMS contains a character foreign to GSM-7: capacity then drops to 70 characters per message.
In brief
UCS-2 is the 16-bit encoding triggered as soon as an SMS contains a character foreign to GSM-7: capacity then drops to 70 characters per message.
Going deeper
It only takes one sign impossible to encode in GSM-7 — emoji, ideogram, Cyrillic or Arabic letter — for the entire SMS to switch to UCS-2, at 16 bits per character.
The effect is mechanical: the 140 bytes of payload now hold only 70 characters. A single emoji in a 100-character text is thus enough to spread it across two SMS, a point to keep in mind for a clean and economical send.
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