For the longest time I've shared an Apple ID with my kids, the idea being to keep control of the iTunes account so they couldn't spend millions on apps, music, and movies.
But recently we've encountered a weird anomaly: sometimes, when someone calls my iPhone, their iPhones ring at the same time! Likewise, if one of them gets a call (which happens pretty much never -- they're teens), my phone rings.
Turns out this isn't an anomaly at all, but rather a "feature" of iOS 8. As I recall, the idea behind it was to allow iPad owners the option of answering calls without having to run for their iPhones, but obviously it's not ideal for anyone sharing an Apple ID with family members.
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.)
I also pinned down the "sometimes" aspect of it: this Continuity/Handoff feature works only when the various devices are in close proximity and connected to Wi-Fi.