RESOLVED: "Facetime and iCloud must be signed in to the same Apple ID to enable Calls on Other Devices."

Issue: When trying to enable "Calls on Other Devices," I still received the "Facetime and iCloud must be signed in to the same Apple ID to enable Calls on Other Devices" error, even after confirming in iCloud settings on each that my iPhone and Mac Book were both signed in to iCloud with the same Apple ID.

Cause: On the iPhone, Settings > FaceTime > "Use your Apple ID for FaceTime" was not enabled.

Resolution: On the iPhone, go to Settings > FaceTime > tap "Use your Apple ID for FaceTime."

Long-winded explanation: Trying to enable Calls on other Devices on my iPhone, I received the error message: "FaceTime and iCloud must be signed in to the same Apple ID to enable Calls on Other Devices."

After confirming that both my Mac Book and my iPhone were signed in to iCloud with the same Apple ID, and that they were on the same WiFi network, I still received the error. I checked the iCloud settings on both to confirm.

I triple checked, and found in the FaceTime settings on my iPhone, even though FaceTime was enabled, "Use your Apple ID for FaceTime" was unchecked.

Once I enabled "Use your Apple ID for FaceTime" in the Facetime settings on my iPhone, and that let me enable the "Calls on Other Devices" setting.