Facetime should work out of the box with most home routers that use NAT and support uPnP. There are various technologies involved that try and get around any firewall or nat setup (see:
http://blog.imtc.org/index.php/2010/06/09/the-technology-behind-apples-facetime- standards/)
The problem i had, was that we have 2 different WiFi networks in our office. A private one, which requires a certificate to use, so is not iPhone friendly, and a public one which has a very restricted firewall. Opening the ports outlined above worked when trying to make a FaceTime call between two phones on the public wireless. When i got home, i phoned a friend who was also at home, and it all worked fine without setup.
Im assuming that both inbound and outbound UDP ports need to be opened because there is direct UDP traffic sent between the devices on the external interface because there is some sort of mediation by apples servers. The outbound phone will send some packets to apples servers to indicate it wants to call the inbound phone, indicating a port that its ready to use, then apples servers find the inbound phone, tell it that a call is requested, at which point the inbound phone will have the 'external' ip address and port of the outbound phone, and will try and establish a direct connection. Since communication is going to be both ways on the external ip, the inbound and outbound port range needs to be open.
Skype only works out of the box, because it defaults to port 80 for communication (have you ever tried to run a web server when skype is running!!), which is generally open on most firewalls.
@blackout00 You will either need to allow the 'To Ip Address' to be any ip (probably * or something similar), or you will have to open up the ports for each of your phones specific IP Addresses (assuming they are not going to change).