You are currently on an Android phone call. To answer this RingCentral call, please end your Android phone call

We are having a problem with a mobile app (the owners phone!). Call comes in after someone dials his extension, cell starts to ring along with desk phone, he goes to answer and the cell continues to ring while getting the message "You are currently on an Android phone call. To answer this RingCentral call, please end your Android phone call." I was able to answer the call after answering the call again but he is not happy with this process. What can be done to fix this.