Asus RT-AC68P with firmware 188.8.131.52.380-3264
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 "Enable" to disable SIP ALG (as of July 2016)
5. Select Apply
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.
***Certain firmware can cause disabling SIP ALG to stop all SIP traffic. It is a bug in the router software. Make sure you have the latest firmware to avoid this issue.***