With Walkie Talkie Mode, users can receive and send voxes handsfree from a bluetooth or wired headset, even if the Voxer app closed and the phone is locked.

Supported Wired Headsets (listen & record - button on the device can be used to send a message when the user has Walkie Talkie Mode enabled):

KOBRA Professional Security Headset Earpiece for iPhone or Android Devices
iASUS 500K Headset with PTT Button
Drift Single-Wire PTT Headset for Voxer

Supported Bluetooth Headsets (listen only - the button will not work on the headset to send a vox back with Walkie Talkie Mode):

Bluetooth headsets that support A2DP (music streaming) and Bluetooth 3.0+ should work for audio playback (listening to messages) but not for sending audio messages (responding).

Plantronics Voyager Legend Wireless Bluetooth Headset 

Jabra Stealth Bluetooth Headset

Plantronics Voyager Focus UC B825 Headset