Yeah, the feature was introduced on iOS 8. Your phone communicates via the local wifi network with other devices signed into the same Apple account on the local network. It's the same way I can make regular phone calls on my iPad - it goes over wifi to my phone.

Apple marketed it as "continuity".

https://www.apple.com/ios/whats-new/continuity/

I've been using it since last fall, to make or receive calls and messages across multiple devices using one mobile phone number.

Thank you for explaining this ^ (and I figured it was something like that) as a few weeks back, I called goalie-parents at their place in Palm Beach on goalie-mom's iPhone and she answered the call on her iPad as both her iPhone & iPad (wifi only) were both connected to their wifi network

That feature is driving me crazy :) My husband has: an iPad, and iPod Touch, a personal AND work iPhone. He linked them all together using iCloud. When they're all in the same place at home it can drive me nuts :)

I have an iPod Touch and and iPhone and DID NOT link them together. I don't need that convenience :)

That link feature is great. It allows me to keep my Android among a family of iDevice users. I could now txt people on their iPads using regular SMS instead of iMessages. My wife has her iPad with her all the time when she is at home, but her iPhone would be sitting somewhere at a corner on vibrate or silence mode. I could have never reached her before the new function.

Be aware that it kinds of drain your iPhone's battery though. Because if you go out with your iPad and iPhone, it creates an instant hotspot on your iPhone to keep the Continuity going.

The use of the SMS may increase the costs for sender and/or recipient. I guess this is a big part of why Whatsapp has the customer base it has. Oh wait, it doesn't work on iPads. Back to Skype.

Mine has never done this. You probably just have your Personal Hotspot feature turned on.

I believe it is called Instant Hotspot and her iPad is set to remember it to autoconnect when it is out of WiFi range. Here is what listed on Apple's Continuity page:

No Wi-Fi? No problem.

If you’re out of Wi-Fi range, your iPad or Mac can connect to your phone’s personal hotspot when your iPhone is close by.* With the new Instant Hotspot feature, you’ll see the name of your iPhone in the list of Wi-Fi networks in Settings on your iPad and in the Wi-Fi menu on your Mac. Just select it and you’re connected. And when you’re not using your phone’s network, your devices intelligently disconnect to save battery life.

Interesting, I didn't know that. Never seen it myself...but I guess I've never looked.