Out of my surprise, The SMS sending feature of Skype under Linux is not supported by its GUI client. I don't know why this feature is left off from the official release. It's quite strange because in its core library, this feature is included. As a consequence, some intelligent hackers use the Skype library interface to write some useful tools for this purpose. At least, I found two softwares that sovle this problem. One is with GUI interface, and another one is a command line tool. What a wonderful world under Linux. You can find what you need and ... multiple choices as always.