if you have any type of high speed internet over 50 Mbps, i would not put your ATA on the WAN, it will choke your bandwidth down to nothing.

As for port forward i use these settings for my 6 line voip phone: 9060-9078 & 11001-12000 UDP. if the 9060-9078 don't work for you for SIP then just use 5060-5061. My accounts are reseller accounts, so yours might not be open to use the 9060 + ports.