1. A method of invoking at least one service application server, in a communication between an origin user agent and a terminal user agent, characterized in that it comprises at least the steps consisting in: opening two separate dialogs, said dialogs comprising messages, between a selective service invocation function and, respectively, the origin user agent and the terminal user agent, the selective service invocation function replacing the terminal user agent in its dialog with the origin user agent and replacing the origin user agent in its dialog with the terminal user agent, at least partly storing the messages originating from the user agents in the two open dialogs, checking at least one conditional service triggering criterion on the two dialogs, and, if the triggering criterion is satisfied, creating at least one third dialog comprising messages between said selective service invocation function and the service application server, correlating said third dialog with the two separate dialogs and at least partly restoring the stored messages to the service application server in order to trigger the invocation of the latter, using the two separate dialogs and said at least one third dialog, correlated together.