Enabling Skype (Note: Only available in the legacy viewer, Naali has a different communications solution based on XMPP. Skype will be possibly enabled in the future)
Enable Skype from 'View' menu -> 'Enable Skype'. Note, that this makes your Skype address available to all other avatars around you.
If Skype is not running already, it will get started. In this case it may take several minutes before Skype can be used.
You will get a notification on the Skype window about realxtend.exe wanting to use it. Allow it.
Making and receiving Skype calls
After enabling Skype in the viewer you can initiate Skype calls to other avatars by clicking on them with right mouse button and selecting 'Skype call' from the pie menu.
You'll get notification from Skype of incoming call if other avatars are trying to call you.
Troubleshooting
'Skype call' choice is dimmed in the pie menu
The other person has not enabled Skype.
No 'Skype call' choice in the pie menu
Make sure Skype window is visible and that you have authorized realxtend.exe to use Skype.
If Skype fails to launch, try starting it manually before enabling Skype in the viewer.
Make sure you are using the latest version of the Skype client.