Cisco highly recommends enabling Dynamic Transmit Power Control (DTPC) on the Access Point (AP) by using the Limit Client Power field in the user interface (or the power client command in Cisco IOS ) to ensure that the APs and Cisco 7920 phones use the same transmit power to avoid one-way audio.
If you do not use a version of the firmware that supports DTPC (such as version 1.0(8) or later) in 7920, configure the Cisco 7920 phones to use the same transmit power as configured on the APs. If the power of the APs varies, set the transmit power of the Cisco 7920 phones to the same level as configured on the AP with the highest transmit power.
Check to see if a firewall or NAT is in the path of the Real-Time Protocol (RTP) packets. By default, firewalls and NATs cause one-way audio or no audio. Cisco IOS and PIX NATs and firewalls have the ability to modify those connections so that two-way audio can flow.
A common AP configuration error concerns Address Resolution Protocol (ARP) caching. The phones expect this option to be enabled on the AP, but it is disabled be default on the AP. For optimal performance, Cisco recommends enabling ARP caching on the AP, especially when using Wi-Fi devices capable of power management. If you use Cisco Aironet cards, ARP caching is enabled by default, but you should ensure that this setting has not been changed by any central client management groups. To configure ARP caching on the AP, select Services > ARP Caching. Issue the dot11 arp-cache command for arp-caching in global configuration mode.
Note: Cisco IOS is required on voice APs because VxWorks no longer add new features that are required for successful voice deployments, such as ARP caching.
Ensure that Temporal Key Integrity Protocol (TKIP) and Message Integrity Check (MIC) (or Cisco KIP and Cisco MIC) are not enabled for the voice VLAN Service Set Identifier (SSID) on the AP. These features are not supported on the Cisco 7920 wireless IP phone.