since the change to pulseaudio it was impossible for me to route my ringing sound to the speakers and the voice/conversation to usb headphones.
Finally I found a solution to this problem which doesn't amount to downgrading to alsa.
If you are looking for a HOWTO on installing usb headsets:
There I also posted the text below.
Thanks to Jim Hunziker for figuring it out!
At first glance (and even at second and third) it seems impossible to set the ringing sound to speakers and the conversation to usb headset.
But follow above HOWTO and then:
1) open pavucontrol and skype>options>sound devices next to each other.
2) press "make a test sound" and (be fast) you will be able to choose (in pavucontrol) the sounddevice to route the ringing sound to, e.g.: Internal audio ananlog stereo.
3) press make a test call and route the call sounds to a usb headset.