For the last 6 months Orange were blocking skype calls over 3G but I just retested now and it seems to be working.
But bear in mind that the Orange 3G data network is at-best rubbish. I would say that over 50% of the time in Zurich city centre it is not functional. Also the underlying data network to the base stations is nothing more than individual ADSL routers from Swisscom, so don't expect much.. Imagine having a few thousand users hanging on your home internet access!
If your phone happens to jump to the 2G network then you are limited to a single voice channel of data (13k max)..they have no EDGE or GPRS network on 2G.
So if you have any intentions of actually using the 3G for anything apart from email and occasional web browsing then stick with Swisscom.
..... 6 months left on Orange contract and counting