Here is some helpful information that will narrow down the lack of side tone for you:
The sidetone function is disabled when the audio cable input connection on the headphones is use.
If the headphones are only connected to a device using the Bluetooth stereo audio profile (A2DP), microphone functionality will not be enabled. This will prevent the user from hearing themselves. In the settings menu of the connected device, be sure the headset profile (HSP) is enabled. Once enabled, unpairing, re-pairing and reconnecting the headphones to the device may be needed.
If more than one device is paired to the headphones, try turning off the device without the phone call. The Bluetooth headset profile (HSP) may be enabled in both devices but one device may not allow the other to operate properly for in-call audio.
If you continue to lack side tone after looking into this information, we encourage you to contact your local Bose support HERE