No. SS7 predates cellphones. It's the legacy control plane for the PSTN (public switched telephone network). It was never designed for security since it originally never crossed corporate boundaries as everyone had to use the monopoly provider. (Except for international calls).
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.
