SIP proxies are elements.The outbound proxy is a normal SIP proxy. You configure your client, the phone or software, to use the proxy for all SIP sessions, just like when you configure your Web browser to use a Web proxy for all Web transactions. In some cases, the outbound proxy is placed alongside the firewall and is the only way to let SIP traffic pass from the internal network to the Internet.
The outbound proxy used by a UA can be automatically set with DHCP (this applies to SIP devices mostly, not SIP softphones). There are also autodiscovery solutions, but not frequently used.
