GSM Character Set
SMS messages use a different character set to standard ASCII.
Characters are 7bits and packed into 140 bytes, giving you up to 160 characters (7 x 160 = 1120 bits, 1120 / 8 bits per byte = 140 bytes).
Conversion to and from various ASCII characters needs to be performed in code when handling SMS messages at the GSM encoded level. An extended character set is also available, but it uses 2 characters per extended character meaning that SMS messages using it can only have up to 80 characters.