See this blog post which explains it all. The trick in Elastix/FreePBX is to create a CUSTOM extension and in the "Dial" setting use this: email@example.com (NO SPACES IN THIS despite how this lousy forum software displays it!!!) That will get you the Skype "echo test" (or their version thereof). To call a different Skype user, repeat but replace echo123 with that user's Skype ID. Calls are limited to five minutes (apparently there's a paid service if you need to make longer calls) but if you exceed that you could always call back (or fire up a real Skype client). But this could be handy for quick calls where you don't want to power up a computer and access a Skype client just for a quick one or two minute call.