Very often this is due to a misconfigured firewall or router. Because the your firewall/router maybe blocking certain traffic, you have no incoming or outgoing audio on calls. If you have a firewall, make sure you are not blocking the UDP PORT 5060-5061 (5060-5063 for a four-line SPA-841) and port for UDP packets in the range of 16384-16482, assuming you have not modified the defaults in the SPA configuration. For help on how to forward ports on your router, visit http://www.portforward.com.


Just as often this is due to your ATA not having its STUN client set properly. STUN works well with the vast majority of routers in the market today, so it is worth checking STUN settings prior to attempting to forward ports. On the SIP page of your web administrator for your SPA confirm that STUN Enable is set to yes and that STUN Server is set to a reliable server, like stun.fwdnet.net:3478.
If you have administrative control of the device, you may also go to the line tab setting; change "Nat Mapping Enable" to yes. On the SIP tab; change "Substitute VIA Addr" to yes, and "EXT IP" to your router's WAN IP.