Cisco 8821 Manual page 86

If wanting to use 2.4 GHz, ensure the 802.11b/g/n network status and 802.11g is enabled.
Recommended to set 12 Mbps as the mandatory (basic) rate and 18 Mbps and higher as supported (optional) rates assuming that
there will not be any 802.11b only clients that will connect to the wireless LAN; however some environments may require 6
Mbps to be enabled as a mandatory (basic) rate.
If 802.11b clients exist, then 11 Mbps should be set as the mandatory (basic) rate and 12 Mbps and higher as supported
(optional).
Cisco Wireless IP Phone 8821 and 8821-EX Wireless LAN Deployment Guide
86