Any Bluetooth headset should work, BT is embedded in the device... We use the Plantronics CS5200 and Jabra Evolve 65 quite a bit, or any Plantronics (Savi 700, Voyager, CS5xx, etc) with an APS-11 EHS cable.
Go to
https://www.plantronics.com/us/en/support/compatibility-guide and select "MITEL" and "6920 IP" for recommended solutions. Jabra has a similar "compatibility" page, but most of their wireless headsets work with DHSG cable (14201-10).
EDIT: I was thinking 6930 which has embedded Bluetooth, but the 6920 does not... Most "UC" Bluetooth headsets work with their associated wireless dongle via the USB port, but make sure they are current firmware for the headset. The electronic hookswitch cables from Plantronics or Jabra work on either model (6920 or 6930).