In most cases there are no special configuration directives to allow SIP, IAX2 devices to operate correctly behind NAT and firewall running in modern SOHO routers.
However there have been interesting issues with Linksys WRT54 routers.
X-Lite SIP phone wasn't working behind generic setup with WRT54 router when it was running on a computer connected over Wi-Fi. It was working fine when computer was connected to Ethernet port. Same issue was found with WRT54GL running OpenWrt firmware.
Problem was solved when QoS was disabled in X-Lite. Go to "Quality of Service" under "Advanced", and set all three items (Audio QoS, Video QoS, Signaling QoS) to "None".
Here is additional information on how to setup QoS for this router: WRT54GL QoS