DHCP on Airport Extreme I have DHCP setup on an Airport Extreme to provide IPs for a network of iMacs and some iPads. 

The Airport's WAN port is connected to a switch that is used for a VOIP phone system. Would the DHCP service on the Airport conflict with the DHCP service for the phone system or will the Airport only provide IPs for its clients on its LAN ports?
In your setup, hooking up the WAN port to the LAN of your VOIP will not cause conflict and will work exactly as you described.

What you have done is create a segmented network on your internal LAN.  The picture below is a good representation of what you have configured.