Incoming FT calls answered on another device continue to ring on my iMac (Yosemite).

Since at least Yosemite, perhaps earlier (I can't remember), whenever I am notified of an incoming FT call on my four devices (iPhone 6, iPad Air, MacBook Air (OS X 10.10.3) and iMac (late 2009 also running Yosemite), if I am away from my iMac and answer the incoming call on my phone or pad, FT continues to ring on my iMac relentlessly whether the app is open or closed until I physically go to it and stop it by opening it or closing it if it is already open in the dock. I assume this is a bug; I do not want to turn it off in case I am in range of the iMac and prefer to answer using that device (even though it is exceedingly slow to open and respond to accept the call). Does anyone know how t fix this?