I found this in another forum:
<snip>
For the Asus RT-N56U there is only one way of disable the SIP ALG function with the latest firmware.
Activate the telnet function of the router and do it by hand.
nvram set nf_sip=0
nvram commit
reboot
That's it the SIP ALG should be disable.
This should solve alot of head ache. Not only sip registration.
</snip>
Is this something I could try? Not sure how to do telnet on the router though.
That information is outdated. Asus has since added the SIP Passthrough option that you already saw - this is what this option is about.