You can use a headset with your DECT Handset.
You can use these types of headsets with your handset:
Headset with a 3.5 mm audio plug
We recommend the Cisco Headset 521 and 522. For more information, see https://www.cisco.com/c/en/us/products/collaboration-endpoints/headset-500-series/index.html.
Bluetooth LE headset
Important Headset Safety Information
High Sound Pressure—Avoid listening to high volume levels for long periods to prevent possible hearing damage.
When you plug in your headset, lower the volume of the headset speaker before you put the headset on. If you remember to lower the volume before you take the headset off, the volume will start lower when you plug in your headset again.
Be aware of your surroundings. When you use your headset, it may block out important external sounds, particularly in emergencies or in noisy environments. Don’t use the headset while driving. Don’t leave your headset or headset cables in an area where people or pets can trip over them. Always supervise children who are near your headset or headset cables.
Beyond physical, mechanical, and technical performance, the audio portion of a headset must sound good to the user and to the party on the far end. Sound quality is subjective, and we cannot guarantee the performance of any headsets. However, various headsets from leading headset manufacturers are reported to perform well with Cisco IP Phones.
For additional information, see https://www.cisco.com/c/en/us/products/unified-communications/uc_endpoints_accessories.html