Cisco 8861 Deployment Manual page 44

If using 5 GHz, it is recommended to enable up to 12 channels only to avoid any potential delay of access point discovery due
to having to scan many channels.
The 5 GHz channel width can be configured for 20 MHz or 40 MHz if using Cisco 802.11n Access Points and 20 MHz, 40
MHz, or 80 MHz if using Cisco 802.11ac Access Points.
It is recommended to utilize the same channel width for all access points.
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 IP Phone 8861 and 8865 Wireless LAN Deployment Guide
44