A method for load balancing within several proxies (Proxy1, Proxy2, Proxy3) in a session Iniatiation Protocol, SIP, SIPN, comprising a SIP user agent (UA'), a domain name server (DNSR), a user database and an access router applying network address translation; characterized in that it comprises the following steps, made by the user agent (UA'): - sending a service record (DNS SRV) request to the domain name server (DNSR) for discovering proxies (Proxy1, Proxy2, Proxy3); - receiving a DNS response comprising a list of service records providing information on discovered proxies (Proxy1, Proxy2, Proxy3); - selecting a proxy among the discovered proxies; - registering at the database of the network (SIPN) by sending a REGISTER request to the selected proxy (Proxy1) via the router, this REGISTER request comprising report parameters in order to enable the user agent (UA') to discover what public IP address and public User Datagram Protocol, UDP, port the router has used as a translation of the private source address and private source port contained in this REGISTER request; - receiving a SIP 200 OK response from the selected proxy (Proxy1) via the router; - extracting, from a Via header field contained in this SIP 200 OK response, the public IP address and the public UDP port that the router has used as a translation of the private source address and private source port contained in the REGISTER request; - and sending a command "AddPortMapping" to the router, by using the (*1), UPNP IGD, protocol, in order to add a port mapping, on the router, for each discovered proxy (Proxy1, Proxy2, Proxy3), this command requesting the router: -- to accept any packet from the corresponding proxy IP address and proxy UDP port, sent to the public IP address and public UDP port of the user agent (UA'); -- and to forward such a packet to the private IP address and private UDP port of the user agent (UA'). (*1) Universal Plug'n'Play - Internet Gateway Device Protocol - sending a service record (DNS SRV) request to the domain name server (DNSR) for discovering proxies (Proxy1, Proxy2, Proxy3); - receiving a DNS response comprising a list of service records providing information on discovered proxies (Proxy1, Proxy2, Proxy3); - selecting a proxy among the discovered proxies; - registering at the database of the network (SIPN) by sending a REGISTER request to the selected proxy (Proxy1) via the router, this REGISTER request comprising report parameters in order to enable the user agent (UA') to discover what public IP address and public User Datagram Protocol, UDP, port the router has used as a translation of the private source address and private source port contained in this REGISTER request; - receiving a SIP 200 OK response from the selected proxy (Proxy1) via the router; - extracting, from a Via header field contained in this SIP 200 OK response, the public IP address and the public UDP port that the router has used as a translation of the private source address and private source port contained in the REGISTER request; - and sending a command "AddPortMapping" to the router, by using the (*1), UPNP IGD, protocol, in order to add a port mapping, on the router, for each discovered proxy (Proxy1, Proxy2, Proxy3), this command requesting the router: -- to accept any packet from the corresponding proxy IP address and proxy UDP port, sent to the public IP address and public UDP port of the user agent (UA'); -- and to forward such a packet to the private IP address and private UDP port of the user agent (UA'). (*1) Universal Plug'n'Play - Internet Gateway Device Protocol -- to accept any packet from the corresponding proxy IP address and proxy UDP port, sent to the public IP address and public UDP port of the user agent (UA'); -- and to forward such a packet to the private IP address and private UDP port of the user agent (UA'). (*1) Universal Plug'n'Play - Internet Gateway Device Protocol