Here’s some troubleshooting you can try:
Forward2Voicemail: You may have activated this function for the specific number. You can turn it off from the "Home" screen or the "Number Settings" screen.
Check App Status: For the app to work as intended, you must allow notifications, remain logged in to your account and keep the app running in the background.
Connection: It my be that you have a bad connection at the time of the incoming call. Turn "Flight Mode" ON and then back OFF to establish a new connection.
Reset App: Force Shuffle to quit by double tapping the the Phone's "Home" button and then swiping the Shuffle preview screen up to close it. Reopen Shuffle and allow for a few seconds to pass before testing an incoming call.