Is unicode not supported in clickatell SMS extension?
I suspect the unicode issue arises from the clickatell extension. It uses the utf8_decode() function to decode the message text from a UTF-8 string to ISO-8859-1 before posting to Clickatell. Hence Clickatell manages UTF-8 messages, the use of utf8_decode() is not necessary, and removing it resolves my issue above.

Replace original code: 

$postDataArray['text'] = utf8_decode(substr($message, 0, 460)); // max of 460 characters, is probably not multi-lingual
$postDataArray['content'] = utf8_decode(substr($message, 0, 460)); // max of 460 characters, is probably not multi-lingual


to this one:

$postDataArray['text'] = substr($message, 0, 460); // max of 460 characters, is probably not multi-lingual
$postDataArray['content'] = substr($message, 0, 460); // max of 460 characters, is probably not multi-lingual


I'll provide a patch to Clickatell extensions, if this solution gets confirmed by the community.