Another thing to try from Obi support

If you have any problems with SIP registration, not being able to receive a call or one-way audio, try to disable the SIP ALG (Application Level Gateway) feature in your router's configuration settings.

The SIP ALG feature available with may off-the-shelf routers is oftentimes not able to translate signaling messages properly such that users may experience registration problems and/or issues with one-way audio.

In these cases, when troubleshooting, it is best to go to your router's configuration and disable SIP ALG.

This wiki article mentions that SIP-ALG functions added in many routers are not properly implemented, hence it is better disable.  www.voip-info.org/wiki/view/Routers+SIP+ALG 

Obihai Support