The most likely reason is to have selected a wrong encoding. For instance, if you are sending text in Chinese characters, Unicode needs to be set. Make sure DCS (Data Coding Scheme) is set to 8. Any other value will result in your text appearing jumbled.
Examples of data coding parameter values are:
Latin alphabet (LATIN-1) - 3
Cyrillic - 6
Hebrew - 7
Unicode - 8
DCS is set to 0 by default which means SMSC Default Alphabet is selected. This encoding looks like the ASCII table (characters 0-127), with the difference that most of the control characters are not present and are replaced by characters from the LATIN-1 upper table (characters 128-255). Thus, this won't be suitable to send non-Latin characters.