It's straightforward outside of an activity, you can see which button accepts or declines the call. During an activity, all I can see is a little popup at the top with the name of the caller, but no indication as to how to take the call. I tried the Start|Stop, Up and Down buttons, but all they do is silence the alert.
I live in Montreal, so I run with a hat on and heavy gloves, so there's no way for me to quickly take the call from my phone or my bluetooth earbuds.