Bose QuietComfort 35 "side tone" not working with iPhone 6

I just bought a Bose QuietComfort 35: a bluetooth headset with noise-canceling. It features "side-tone" for telephone calls, which supposedly allows you to hear your own voice through the headset when talking during a call. However, I can't hear any difference when calls come in; although the caller can hear me just fine, I can't hear my own voice. I read that the issue might relate to the headset profile (i.e., the iPhone not switching from A2DP, for stereo music, to HFP - handsfree protocol - when the call occurs). My question is this: do you own the QC 35? If so, does the side-tone feature work on yours? And if so, can you provide me with any information about how to make it work?