Without controlling the OS nor the applications, it's a tough task, most things are a band-aid. The possibility of an application firewall evokes me mixed feel

Without controlling the OS nor the applications, it's a tough task, most things are a band-aid. The possibility of an application firewall evokes me mixed feelings: it'd be great, but I'd have to set what's allowed per app, per phone. Even at that point I doubt it's possible to assure a 100% rate.