Versions: Ubuntu 15.10, Skype 4.3.0.37
In Skype->Options->Sound Devices I want Ringing=rear speakers and Speakers=headphones, for, I think, obvious reasons.
However, there is only one device permitted for all three, Microphone, Speakers and Ringing:
PulseAudio server (local)
Unity's sound "indicator" at the upper right of the desktop has a dropdown to "Sound Settings" that I can use to redirect PulseAudio's audio streams for various devices, but that doesn't let me direct one output stream (Skype's "Ringing") to one device and another output stream (Skype's "Speaker") to another device.