Definition
Unicode SMS is a text message sent with UCS-2 encoding so it can contain characters outside the GSM-7 alphabet, such as Arabic script or emoji. It holds 70 characters per single SMS and 67 per segment.
Any message containing even one non-GSM-7 character is sent as Unicode in its entirety. That includes Arabic, Hindi, Chinese, most emoji and some typographic characters copied from word processors.
Because the capacity drops from 160 to 70 characters, a Unicode message costs more segments. A 200-character Arabic message needs 3 segments; the same length in GSM-7 needs 2.
In the GCC, Arabic SMS is standard for government, banking and retail communication, and providers must support UCS-2 end to end, including correct right-to-left display on handsets.
Tip: if you send English and Arabic versions, send them as separate messages so the English one stays in cheaper GSM-7, and check messages for stray Unicode punctuation.