The invention relates to a method for data exchange between at least one calling network element and at least one network element to be called, in different first and second network areas separated by network node devices or firewalls. A network address which is locally valid only in the respective network area is generally associated with the network elements in said separated network areas or domains. Said locally valid network address is converted into message header entries, which send the network elements to network elements localised outside the network region, by the respective network node device, by means of a network address which is valid outside the respective network area, especially a globally valid network address of the network node element. According to the invention, following the emission of an invite message (e.g. an SIP invite), the content of the invite message is modified on the basis of the network address which is contained in the message header and valid outside the first network area, e.g. an entry of the IP address contained in the header and previously modified by the network nodes, in a body of the message. The second network element to be called then sends a message towards the first calling network element. Said message generates a continuous filter or pinhole in the second network node device and is dropped on the first network node device. A confirmation message is sent by the network element to be called. Said confirmation message is provided, for example, by the used communication protocol, for example SIP, and is modified analogously to the invite message, for example on a switch or network node arranged between the network node devices. Analogously, a message generating a continuous filter for the first network node device is sent by the network element to be called once the confirmation message has been received. In this way, an RTP connection (Real Time Protocol) can be established between the calling network element and the network element to be called, without any further consideration of the address conversion NAT between the two communicating network elements.