Having SIP enabled (or checked) can affect things like Vonage and other VoIP softwares.... For me I had issues sending & receiving FAXes until I unchecked the SIP box.... Because the router was blocking FAX-related data packets going into my vonage adapter that is behind my 655.... Soon as I unchecked SIP, saved and rebooted the 655....I could FAX no problems.
As others have said, you def want SPI enabled in the firewall settings of the setups.