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.
5. 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.

Older firmware before 3.0.0.4.374_257 SIP ALG --
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

Reboot
Voilà

Then reboot the router for the changes to take effect.

NOTE: The information provided above is from another OnSIP customer offering these settings for other customers with a similar device. OnSIP does not sell nor monitor equipment and/or it’s firmware updates/etc. The settings of routers can change and are out of the control of OnSIP. For best results, reach out to the manufacturer directly. 

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