Configure your router to port forward "TCP port 80" to the IP address currently being used by your Linksys ATA device. If you do this often, we suggest that you use static IP address for the ATA device, instead of DHCP. (For help with port forwarding, consult your router documentation.)
On the EXT tab of the administration web server, change the value of Nat Mapping Enable to yes. On the SIP tab; change Substitute VIA Addr to yes, and the EXT IP parameter to the IP address of your router.
Make sure you are not blocking the UDP PORT 5060,5061 and port for UDP packets in the range of 16384-16482. Also, disable "SPI" if this feature is provided by your firewall. Identify the SIP server to which the ATA device is registering, if it supports NAT, using the Outbound Proxy parameter under the EXT 1 of the SPA-901.
Add a STUN server to allow traversal of UDP packets through the NAT device. On the SIP tab of the administration web server, set STUN Enable to yes, and enter the IP address of the STUN server in STUN Server. STUN (Simple Traversal of UDP through NATs) is a protocol defined by RFC 3489, that allows a client behind a NAT device to find out its public address, the type of NAT it is behind, and the port associated on the Internet connection with a particular local port. This information is used to set up UDP communication between two hosts that are both behind NAT routers. Open source STUN software can be obtained at the following website:
Note: STUN does not work with a symmetric NAT router. Enable debug through syslog (for instructions, click here), and set STUN Test Enable to yes. The messages indicate whether you have symmetric NAT or not.