Aswath points to a white paper by Connectotel that describes server-side Skype applications, and notes that one could use the techniques described to build competitors to SkypeIn and SkypeOut (and, I'd add, Skype Voicemail).
Alas, playing in this sandbox has some risks.
For one thing, there are limitations. To quote one section of the Connectotel white paper:
It is important to be aware that granting authorization within the Skype client is currently a manual process for which no APIs exist. This implies a potentially large amount of work for personnel who will have to grant or deny authorization one-by-one to the userswho request it.In other words, authorization can't be easily automated, which makes it rather hard to scale. This limitation of the API points out the second risk: If you do something with the API that starts taking money out of Skype's pockets, Skype can always change the API. After all:
Skype, in its sole discretion, reserves the right to add additional features or functions, or to provide programming fixes, updates and upgrades, to the Skype Software. You acknowledge and agree that Skype has no obligation to make available to You any subsequent versions of the Skype Software. (Skype EULA)And
Furthermore, you acknowledge and agree that Skype, in its sole discretion, may modify or discontinue or suspend Your ability to use any version of the Skype Software, or terminate any license hereunder, at any time. Skype also may suspend or terminate any license hereunder and disable any Skype Software You may already have accessed or installed without prior notice at any time. (Skype EULA)Not something I'd build a business case on.