Kannel, open source SMS/WAP gateway, is internally using UTF-8 encoding from stable release 1.4.2 and on.
Nevertheless there is the chance that you may want to send a message with special preprocessing. That is when you want to use the GSM 7 bit encoding for communication with the SMSC and the message you want to send contains unmappable UTF8 characters to the GSM encoding.
The code preprocessing mentioned above, should contain maps from the umpappable UTF8 character codes to other UTF8 character codes that are available to the GSM 7 bit alphabet. For example if you try to feed Kannel with a message containning the non capital greek letter omega ‘?’ you should expect the default unmappable character sighn ‘?’ as the final output. What you should do, prior sending the message to kannel, is to map the non capital omega character with the capital omega ‘?’ (03C9 – UTF8 encoding) character that is available to the GSM 7 bit encoding.
More information about the GSM 7 bit encoding can be found at http://en.wikipedia.org/wiki/GSM_03.38