gistrar server: the SIP proxy server asks the registrar server about the type of the called and the caller parties. The SIP proxy server decides to invoke ALG in case the two involved parties are from different realms, i.e., IPv4-only and IPv6-only. - The second alternative is to examine the atypes tag as conveyed in the INVITE request and ask the registrar server about the type of the called party. In this scenario the SIP proxy server routes the call by comparing the compatibility of the two retrieved values (IP connectivity types of called and caller parties). For SIP user agents that don't support atypes attribute, the SIP service plat- form generates an OPTIONS method destined to these SIP user agents. The purpose of this procedure is to check the capabilities of the SIP user agents and to drive the call optimization process. Once receiving the OPTIONS mes- sage, a given SIP user agent must answer with a "200 OK" message enclosing its network and media capabilities. Based on the content of this answer, the service platform will be able to classify its attached SIP user agents according to their network capabilities. 3.4.2 Blank SDP Procedure The procedure described hereafter has been designed in order to improve IP versions coexistence (i.e., manage service platform IP heterogeneity) with- out requiring deep modifications of the involved service nodes. In this aim, this procedure presents the advantage of relying on standard SIP messages, thus allowing a simple and faster deployment into operational networks. The proposed procedure is suitable for service providers deploying intermediate nodes to hide the service core elements from their customers. The first service element visible to end users is one of these intermediate nodes. In such as case, driving the call routing process and therefore optimization the media path shall take into account the presence of three call legs: called SIP user agent until the intermediate node (denoted as originating SBC), leg between the originating SBC and the terminating SB