With the help of a key management protocol, the transmitted key information is authenticated by at least one certificate signed by the terminals, and at least one fingerprint of the public keys or certificate, which were used for authenticating the key information, is added to the useful part of an SIP message. The identity information present in the header of an SIP message is additionally copied into a region of the header or the useful part, and a signature is produced by way of the fingerprint, the datum information presented in the header of an SIP message, the copied identity information, and optionally the certificate reference information, and is inserted into a further region of the header of the SIP message. The additional signature that is produced and inserted can remain uninfluenced during a transmission across several networks of different network operators.