Able to answer—but not make—iPhone calls on Mac

Background

Among the many promising features of OS X 10.10 grouped under the label "Continuity" is the ability to make and answer iPhone calls from the Mac. The Mac uses the iPhone's connection to the carrier's voice network. I think this technology works over wifi.

Problem

I'm able to answer calls, but not make them. The error message I receive on my Mac is, "iPhone calls not available. Your iPhone must use the same iCloud and FaceTime account."

What I've done

I've confirmed that both iCloud and FaceTime on my iPhone—as well as iCloud on my Mac—are logged into the same account.

Question

What could be causing this issue, and what should I do to fix it?