FYI is a mixed success when using the GSM Extended Char Set (GSM 03.38), where a subset of special characters uses 2 x 7 bit characters, but the main characters are 1 x 7 bits per Char (see long-sms-text-messages-and-the- 160-character-limit for a good explanation).
Messages with square brackets [] (GSM 03.38), routed through one carrier, are correctly decoded on the iPhone, but the square bracket + next letter are scrambled when received through another carrier (also iPhone). It seems that it depends on the operator and is not specifically related to the iPhone, because the problem is also observed on Android.
Therefore, be careful using the Extended Char Set.