The invention concerns a method for invoking at least one service application server (AS) during a communication between an originating user agent (O_UA) and end user agent (T_UA). It consists at least in opening (A) two separate dialogues, between a service selecting function (SIF) and, respectively, the originating agent (O_UA) and the end agent (T_UA). The function (SIF) is substituted to the user agent (T_UA) in its dialogue with the user agent (O_UA) and to the user agent (O_UA) in its dialogue with the user agent (T_UA). The messages from the user agents during the two opened dialogues are stored (B) and at least one service conditional triggering criterion on the two dialogues is verified (C). If the triggering criterion is verified, at least part of the messages is restored (D) to the server so as to trigger the invocation thereof. The invention is useful for invoking application servers in a SIP network.