SIP2SIP

This is the minimum config for Cisco SPA525G that allows me to send and receive SIP2SIP calls from behind a residential NAT.

Go to Voice / Ext 1 (or whatever extension it is) / advanced
        - NAT mapping enable: No
        - Proxy: sip2sip.info
        - Outbound proxy: proxy.sipthor.net
        - Use outbound proxy: yes (otherwise, won't register)
        - User id: just your username (don't include @sip2sip.info)
        - Use auth id: no
        - Use DNS SRV: yes 
        - DNS SRV auto prefix: yes
        - Register: yes (if you want to receive calls)
        - Make call without reg: yes (if you want)