Why do my received calls go mute if the headset cable is added to the audio port?

[Windows Central Question]

Received call goes mute if headset cable is added to audio port

My wife's Lumia635 from CricketWireless works ok if calls are taken without a headset cable. But, if a call is received and taken, adding a headset cable into the audio connector port mutes the call.

Workaround is to soft reset the phone with the headset cable on and to call the same number back.

Is there a known workaround for the issue? Phone may be running WindowsPhone 8.1.

Appreciate any responses. And thanks enormous in advance for sharing your skills and answering this question.

Cheers,
Cal