Hi Thomas! First, the definition from RFC 3261: Typically, SIP messages are sent to the SIP proxy which is responsible for this domain. that means if the request URI is sip:klaus@iptel.org, the meassage will be sent to the SIP proxy of iptel.org. In same cases, it is requiered that the SIP messages passes a certain SIP proxy. E.g. in enterprise environments with certain security policies all SIP messages will be controlled by the company's SIP proxy before they will leave the corporate network. Another scenario is NAT traversal. If the iptel.org's SIP proxy wouldn't do NAT traversal, you can insert a certain SIP proxy into the signaling path, which is responsible for NAT traversal. In this cases, this additional SIP proxies will be called Outboundproxy. If a SIP UA is configured with an outboundproxy, all SIP requests will be sent to the outboundproxy instead of sending it directly to the SIP proxy indicated in the request URI. In certain cases (NAT travesal, security ...) the outboundproxy also rewrites the SDP in the SIP messages to direct audio to an RTP proxy. regards, Klaus support wrote: