Why am I unable to switch facetime audio calls between my mac and iphone via handoff?

Hi all,
I was on a facetime audio call on my mac and realised I can't switch the call to my phone from the mac like I can with an ordinary phone call.
I'm wondering why this is? Of course, it's lovely to have the advantage of using the phone and being able to use siri on the phone etc, but if I was on facetime on the mac and wanted to switch to the phone because I was going out and didn't want to end the call and call back, it would be nice to find the touch to return to call on the iphone screen and switch over seamlessly.
I would welcome any thoughts on this.