If Fring is running in the background, when someone calls you the chat engine icon pops on screen and you answer the call as normal, sound comes through the earpiece as normal... it's a cool tool because it looks like you're taking a standard call
Incidentally I downloaded Skype Mobile 2.2, only to learn that it's not compatible with
Hermes 
Apparently it's still workable as a text chat but then what's the point? Fring allows me to 'subscribe' to several clients including Skype and I can now make voice calls on their service!