Configuring AirPort Extreme with Existing Router
The airport extreme should be connected as just a device with the cisco router handling DHCP and NAT.  in the airport configuration go to the Network Tab and click the double headed arrow under Router Mode set it to "Off(Bridged Mode)


You can then go to Wireless and set that how ever you want all traffic will go to the cisco router and either be forwarded to the Internet or go to the voip, printer, etc.