The server supports two modes: SA (stand-alone) and AS (application server). These modes are not exclusive and no special configuration is needed.
The SA mode allows any SIP client to directly connect to the system without any intermediate node. If the client requires to be registered then, you can enable this option as explained here because the SIP registrar mode is OFF by default.
The AS mode is useful if you already have your own SIP network/server and want to integrate the Telepresence system as an application server. This mode has been tested against Asterisk, reSIProcate, openSIPS and Kamailio. This is as easy as forwarding any INVITE (based on some criterias, e.g. name equal to @conf-call.org) received by the SIP registrar to the Telepresence system.