SMS Character Sets
When you want to send an SMS message, whether by itself or as part of a structured SMS or iSMS activity, you'll need to understand that there are two different sets of characters that are used by different mobile carriers:
1 - GSM
Many mobile networks limit text messages to the 128 character GSM character set (full list here). When using this set, a text message can contain 160 characters. This means there can be 160 characters in an SMS.
There are also 10 "extended GSM" characters, which use two 7-bit characters (ESC followed by another character). An example is the Euro symbol: €. If you use any extended GSM characters, it will take up two, not one, of the 160 character limit.
2 - UCS2
If you need to send characters that aren't in the GSM character set, you'll need to use UCS-2 character set. UCS-2 allows you to send most common characters that you could type on your computer (which uses unicode character set), but each UCS-2 character takes up twice as much memory as a GSM character. That means that if you use any UCS-2 characters (full list here), your text message can only include 70 characters, not 160.