A typical "Skype User" is composed of alphanumeric characters, for example "skip2pbx".
To be able to call a Skype customer directly from a traditional telephone you must use the speed-calling feature of the Web interface, associating a short numerical number (Example. "23") with the Skype account that it would be assigned to call (Ex. "skip2pbx").

Now, simply by typing on your telephone the number 23 you will call the Skype account.