No audio is usually caused by the RTP ports being blocked by the firewall.
If you have made changes to the firewall you might want to reset it back to defaults because the standard settings on consumer routers usually work for VoIP.
Other things to check:
Disable SIP_ALG
Make sure the blue port on the Obi (if it is an ATA and has two ports) is connected to the routers LAN port
If it's an Obi phone connect the SW port to the routers LAN port