Glossary
What is GSM-7?
GSM-7 is the standard 7-bit character set for texts, covering plain letters, digits and common punctuation, so one SMS part can hold 160 characters.
Each character takes seven bits, which is how 160 of them squeeze into the 140 bytes an SMS has room for. The set covers A to Z in both cases, 0 to 9, everyday punctuation, the £ and @ signs and a handful of accented letters such as é and ü. A few more characters sit in an extension table and take two places each: ^ { } [ ] ~ | \ and the euro sign.
What catches people out is text pasted from a word processor. A Manchester gym's 'Class moved to 6pm tonight, see you there' is plain GSM-7 if it's typed on a keyboard. Copy the same line out of a document with a curly apostrophe in it and the whole text switches to Unicode, because the curly version isn't in the GSM-7 set at all.
CaptiFi Connect sends in GSM-7 whenever every character in the text has a GSM-7 form, and the composer shows which characters are stopping it so you can swap them for plain ones. Extension characters are counted as two, and a part is never split halfway through one, so now and then a part holds one character fewer than you'd expect.