The invention relates to a method and system for booking a reservation in a booking system and synchronizing bookings in several booking systems comprising at least one booking systems; at least one service provider; a mediator service; a client, and at least one client terminal device that can be a mobile device and that includes a dialogue. In addition, the system comprises telecommunication connections that are used to connect the booking systems, the service providers, the mediator, and the client terminal device. The client uses the dialogue to enter information into the system. There mediator gets inquiries and answers from booking systems, service providers, and dialogues, and transfers and adapts information between them.