After testing this device using firmware version 188.8.131.52 we have determined that it will not work with OnSIP at this time. The Router would not allow SIP packets to pass through to our SIP Proxies in any form.
If there is a SIP ALG running, telnet into the device and enter 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.