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T-Mobile was the first company to push Wi-Fi calling, a way to connect calls from a cellular phone over a Wi-Fi network to their calling system. The company offered unlimited domestic calls with an add-on service and eventually rolled it into its general plans. This was a practical and competitive move: T-Mobile hadn’t yet built out a robust national network, while now it’s often ranked the best in testing.

But Wi-Fi calling persisted and spread to other carriers. It’s offered by most U.S. and Canadian wireless phone companies and many others around the world. While on most carriers there’s no financial benefit, like reduced charges or avoiding plan add-ons, you simply get a better-quality call in indoor locations with weak cellular service and a good Wi-Fi network.

You can turn on Wi-Fi calling via Settings > Phone > Wi-Fi Calling. The option appears only if your carrier offers it for your account.

However, you might notice there’s a switch at the bottom of Wi-Fi Calling: Add Wi-Fi Calling for Other Devices. This lets you use FaceTime on all the other devices connected to your iCloud account—iPads, Macs, and even other iPhones that don’t have an active calling plan.

If that switch isn’t turned on, you might further observe that you may be unable to tap to enable it—it might even taunt you, appearing to turn on and then turning itself suddenly off. This bedevils people because it’s not obvious what to do next. Here’s what to do:

If you don’t tap or click in step 5, you can enable Wi-Fi Calling:

Calls on Other Devices and Wi-Fi Calling differ in this way:

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Glenn Fleishman’s most recent books include Take Control of Untangling Connections, Take Control of Your M-Series Mac, and Take Control of Securing Your Mac. In his spare time, he writes about printing and type history. He’s a senior contributor to Macworld, where he writes Mac 911.

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