I admittedly don't know that much about it, but the googling I did indicated the SS7 is only active when you roam/call to legacy networks with 2G/3G that aren't all-IP?
From what I've read that tends to change when the phone companies in question have shut off 2G and 3G though. The only reason to keep using SS7 is for intercompatibility with legacy networks who still have 2G or 3G devices on them.
