It should activate any time you’re in a phone call, or otherwise using the microphone. When you’re not using the microphone, you can’t turn on the sidetone. It’s automatic, like most things with Bose headphones. There’s no way to turn it on unless you’re using the mic.
I use the headphones with an iPhone 6 Plus for phone calls, and it has also worked for me on a PC and a Mac during online chat.
You might also try downloading the Bose Connect app to check for the latest firmware. I also find that to be an easy way to manage connections.
Thanks for reading, hope that helps!