Text messages allow 140 bytes per message. SMS uses a special 7 bit character set known as "GSM" to fit 160 characters into 140 bytes.
SMS messages are 160 GSM characters per message; if a sent message exceeds that, the message will split into multiple messages with a (1/2) (2/2) identifier included.
Including some uncommon ASCII characters that aren't included in the GSM character set will reduce the message length to 14o characters. The @ symbol is one of the more common culprits here.
Including a multibyte character (such as a Japanese, Chinese, Korean, or Arabic character) will further reduce the characters per message. A multibyte character occupies two bytes, so a message containing a multibyte character will only allow 70 characters in your 140 byte message.
If Tropo splits a message into multiple messages, each message sent will be billed. In other words, sending a 165 character message will be billed as two messages.