Cisco 9971 Deployment Manual page 67

Ensure that channel 165 is not enabled in the DCA list as the Cisco Unified IP Phone 9971does not support this channel.
If using 2.4 GHz, only channels 1, 6, and 11 should be enabled in the DCA list.
It is recommended to configure the 2.4 GHz channel for 20 MHz even if using Cisco 802.11n Access Points capable of 40 MHz
due to the limited number of channels available in 2.4 GHz.
Cisco Unified IP Phone 9971 Wireless LAN Deployment Guide
67