Asus RT-AC66U devices with their most current firmware enables their SIP ALG by default. THERE IS NO GUI OPTION TO DISABLE IT
So as to add to your knowledge base, I thought I'd share the solution:
To disable the SIP ALG manually, you enable telnet to the device via the WWW interface
Telnet to the device (from a command line enter "telnet 192.168.1.1" or the appropriate IP address for the device.)
Issue the following commands:
nvram get nf_sip
(It should return a "1")
nvram set nf_sip=0 nvram commit
Then reboot the router for the changes to take effect.
Under firmware 188.8.131.52.374_257 SIP ALG is located in (via the web interface):
1. Log into the router's web interface.
2. Go to Advanced Settings / WAN on left side.
3. From the tabs across the top, choose NAT Pass through.
4. Change SIP Pass-Through to "Disable" in order to disable the SIP ALG.
For phones to pick up the change immediately, reboot each of them, otherwise they will pick up the new NAT table with changes during their next registration.