Fortunately, you can turn it off. Here’s how:
Step 1: Starting with your own iPhone, tap the Settings icon, then scroll down until you find FaceTime.
Step 2: Tap it, then disable iPhone Cellular Calls.
Step 3: Repeat the process on every other iDevice that’s ringing when it shouldn’t be.
Presto! Calls will now go only to the phone number that was dialed. At least, that was my outcome. Your mileage may vary, of course, as there are some additional iOS settings that can affect the way calls and apps are shared. (There’s at least one Apple discussion thread devoted to this very topic.)