An apparatus for interconnecting a user agent to a cluster of servers, the apparatus comprising: - a first protocol agent configured to exchange first messages pertaining to a SIP session with said user agent according to a, Session Initiation Protocol, SIP protocol, wherein said apparatus has the role of a User Agent Server; - a second protocol agent configured to exchange second messages pertaining to said SIP session with a selected server from among said cluster of servers according to the SIP protocol, wherein said apparatus has the role of a User Agent Client; - interconnection means for relaying messages from among said first messages and said second messages between said first protocol agent and said second protocol agent; wherein said second protocol agent is further configured to terminate elasticity control messages received from said cluster of servers, wherein said apparatus is adapted to continue said relaying as long as said SIP session requires it, and wherein said apparatus is configured to update said interconnection means in accordance with said elasticity control messages during said SIP session; said apparatus being configured, in response to a REGISTER request received from the user agent, to: update the REGISTER request to replace a contact address of the user agent with its own address and publishing itself as a contact on the client's behalf; store the contact address of the user agent locally or encoding this address in an updated contact header pertaining to said REGISTER request; add a VIA header and a RECORD-ROUTE header to the REGISTER request, so as to make sure responses and subsequent requests will pass the apparatus as well; and/or said apparatus being configured, in response to an INVITE request received from the user agent, to: add a VIA header and a RECORD-ROUTE header to the INVITE request, so as to make sure responses and subsequent requests belonging to the same SIP dialog will go through the apparatus; and/or said apparatus being configured, in response to an INVITE request received from a server among said plurality of servers, to: add a VIA header and a RECORD-ROUTE header so as to make sure responses and subsequent requests belonging to the same SIP dialog will go through the apparatus. - a first protocol agent configured to exchange first messages pertaining to a SIP session with said user agent according to a, Session Initiation Protocol, SIP protocol, wherein said apparatus has the role of a User Agent Server; - a second protocol agent configured to exchange second messages pertaining to said SIP session with a selected server from among said cluster of servers according to the SIP protocol, wherein said apparatus has the role of a User Agent Client; - interconnection means for relaying messages from among said first messages and said second messages between said first protocol agent and said second protocol agent; update the REGISTER request to replace a contact address of the user agent with its own address and publishing itself as a contact on the client's behalf; store the contact address of the user agent locally or encoding this address in an updated contact header pertaining to said REGISTER request; add a VIA header and a RECORD-ROUTE header to the REGISTER request, so as to make sure responses and subsequent requests will pass the apparatus as well; add a VIA header and a RECORD-ROUTE header to the INVITE request, so as to make sure responses and subsequent requests belonging to the same SIP dialog will go through the apparatus; add a VIA header and a RECORD-ROUTE header so as to make sure responses and subsequent requests belonging to the same SIP dialog will go through the apparatus.