Method for configuring access by a user (300, 302) to fallback communication services accessible via a transport network, the fallback services providing a minimum communication service to the user (300, 302) when the transport network has lost communication with a corresponding core network, the access to the fallback communication services requiring an authentication, the method comprises the following steps: - an application client hosted on a user equipment sends an authentication request to an identity management server,; - in the event of rejection of the authentication request by the identity management server,: • the identity management server allocates a unique fallback identifier to the user and sends the fallback identifier to the application client,; • the application client sends a request for fallback user profile comprising the fallback identifier to a configuration management server and receives a corresponding fallback user profile sent by the configuration management server, the user is then authenticated in "fallback mode"; • the application client sends a request for fallback multimedia group profiles comprising the fallback identifier to a multimedia group management server and receives at least one corresponding fallback multimedia group profile sent by the multimedia group management server,; • the application client sends a request for fallback multimedia group encryption keys and private communications encryption keys comprising the fallback identifier to an encryption keys management server and receives at least one fallback multimedia group encryption key and at least one corresponding private communications encryption key sent by the encryption keys management server; • once the application client has received the fallback user profile, the fallback multimedia group profile(s), the fallback multimedia group encryption key(s) and the private communications encryption key(s), the user can access the fallback communication services, each fallback multimedia group providing group communication fallback services, wherein the members of a fallback multimedia group are all authenticated "fallback mode" users. - an application client hosted on a user equipment sends an authentication request to an identity management server,; - in the event of rejection of the authentication request by the identity management server,: • the identity management server allocates a unique fallback identifier to the user and sends the fallback identifier to the application client,; • the application client sends a request for fallback user profile comprising the fallback identifier to a configuration management server and receives a corresponding fallback user profile sent by the configuration management server, the user is then authenticated in "fallback mode"; • the application client sends a request for fallback multimedia group profiles comprising the fallback identifier to a multimedia group management server and receives at least one corresponding fallback multimedia group profile sent by the multimedia group management server,; • the application client sends a request for fallback multimedia group encryption keys and private communications encryption keys comprising the fallback identifier to an encryption keys management server and receives at least one fallback multimedia group encryption key and at least one corresponding private communications encryption key sent by the encryption keys management server; • once the application client has received the fallback user profile, the fallback multimedia group profile(s), the fallback multimedia group encryption key(s) and the private communications encryption key(s), the user can access the fallback communication services, each fallback multimedia group providing group communication fallback services, wherein the members of a fallback multimedia group are all authenticated "fallback mode" users. • the identity management server allocates a unique fallback identifier to the user and sends the fallback identifier to the application client,; • the application client sends a request for fallback user profile comprising the fallback identifier to a configuration management server and receives a corresponding fallback user profile sent by the configuration management server, the user is then authenticated in "fallback mode"; • the application client sends a request for fallback multimedia group profiles comprising the fallback identifier to a multimedia group management server and receives at least one corresponding fallback multimedia group profile sent by the multimedia group management server,; • the application client sends a request for fallback multimedia group encryption keys and private communications encryption keys comprising the fallback identifier to an encryption keys management server and receives at least one fallback multimedia group encryption key and at least one corresponding private communications encryption key sent by the encryption keys management server; • once the application client has received the fallback user profile, the fallback multimedia group profile(s), the fallback multimedia group encryption key(s) and the private communications encryption key(s), the user can access the fallback communication services, each fallback multimedia group providing group communication fallback services, wherein the members of a fallback multimedia group are all authenticated "fallback mode" users.