To Pair your Bluetooth headset with your Cisco 8851 VOIP phone:

• On the Cisco 8851 phone, hit the Applications options key that looks like a gear.
• Select “Bluetooth” from the menu. 
• Select “Add bluetooth device”. 
• Put your Bluetooth device in discovery mode. 
• Once the Cisco 8851 set finds the phone, hit the “Pair” softkey.  
• Once it is paired, hit “Exit” to exit the Bluetooth menu and “Exit to exit the Applications options menu. 
• You should now be able to make a call on your Cisco 8851 and use your Bluetooth device instead of the handset or speakerphone. 

To disconnect your Bluetooth headset from your Cisco 8851 VOIP phone temporarily or permanently:

• Hit the Applications options key that looks like a gear.
• Select “Bluetooth” from the menu.
• Navigate to your Bluetooth device from the menu.
• Hit “Delete” to remove it permanently from the Cisco phone or hit ‘Disconnect” to temporarily disconnect it.

https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cuipph/8800-series/english/user-guide/P881_BK_C3A802A0_00_cisco-ip-phone-8800-user_guide/P881_BK_C3A802A0_00_cisco-ip-phone-8800-user_guide_chapter_0111.html#P881_TP_B0A0E954_00