They are billing this as a "one number" service, that is, one number per handset. I don't see that the conventional voice line, if you keep it, would interact with this system at all. Their installation instructions show that a normal aDSL filter is used at the wall, so the Orange box won't even see it. (actually, looking at their FAQ, Orange do acknowledge that this is what will happen.)
For the UMA handsets (if they are any good), call traffic should be flipped between the local WiFi box and the outside cell network transparently, just as if it was a jump between towers.