Dialing from MacBook shows that my iPhone is already on a call

When using the new continuity feature in OS X Yosemite together with my iPhone 5S running iOS 8, I can't dial from my MacBook. Here's what I do:

• Both devices are on the same Wifi network


• Both devices are logged in using the same iCloud/Apple ID


• On the MacBook, I open FaceTime and type a phone number into the search field


• Next, I click the phone icon next to the number and select the "Call Using iPhone option"


• A popup is shown indicating that the phone is dialing the number

After a couple of seconds, the following message is shown:

The message says: "iPhone in Use. Your iPhone is already on a call." I'm pretty sure that my phone is not on a call at this point.

Clicking the "Try Again" option provides the same result.

What am I doing wrong?