Here are some of the recommended configurations to consider that are specific to ZD and APs 
 

1. Dedicated SSID for voice clients. Use a dedicated VLAN as well.

2. Tunnel traffic to ZD

3. Enable Call Admission Control (CAC)

4. DTIM value of 2 (this will ensure decent battery life without compromising call quality)

5. Enable OFDM-only

6. BSS minrate of 5.5 Mbps

7. Use particular channels only (e.g.: channels 1, 6, and 11 only)

8. Enable 802.11d

9. Increase background scan-interval to a higher value - 5 or 10 minutes

10. See if ChannelFly is appropriate for your environment. If this doesn't suit, switch to background scanning and increase the scan interval to a higher value as mentioned above.

11. Disable mesh

12. Avoid any additional load on the APs

13. Enable RTS/CTS

14. Eliminate rogue APs and mobile hotspots

Please note that not all these settings may be appropriate for your deployment and VoIP handset you are using. Use only the ones that are recommended by your VoIP handset maker.

 

Here are some of the configuration settings to consider that are specific to a VoIP handset: 

1. Enable voice DSCP marking

2. Operate at max transmit power

3. Prefer 5 GHz frequency mode if available. This might require more number of APs. So site survey is the key.

4. Disable DHCP request on roaming if there is a single VLAN for all voice clients

5. Control roaming if the device permits fine tuning
6. QoS preference. Some handset makers recommend using Continuously Awake Mode (CAM). Some may recommend legacy power save.

Find the latest VoIP design best practices for SmartZone 3.6+ at This Link.