My app will look like this: two people enter a chat or something else in a page and have a skype name or Gtalk. Is it possible to make an anchor to call a voice client from the web page? Is there a flash fallback for this?
I know the skype: prefix that actually doesn't work for me in Ubuntu, but should work for Windows users.
I primarily target PCs & macs, but supporting mobile platforms with a solution will be nice.
For Downvoters: any comments why you do this? Please bother to explain me the obvious thing that I don't know and make this question.
Your question is much too complex. Instead of answering it, which would take several pages, the person might as well write the application him/herself and sell it.
You have to divide your problem in tiny bits, and have a question for each one.
You could at least start with looking at gTalk API and Skype API to see what you need yourself.