SIG ALG and consumer-grade routers tend to be more problematic. More than once I have had to troubleshoot a VoIP system connected to a Nighthawk router. These also tend to have SIP ALG on by default. I had to factory reset the last one for the phones to work. Other times I had to power cycle them.

A handy tool to detect SIP ALG:
http://mcstest.visualware.com/myspeed/myvoiph5_g711_sipalg.html