Abstract: A booster arrangement for a portable telephone, which includes a telephone (2) and booster (1), and which booster receives identity information from the portable telephone, is characterized in that it incorporates a security procedure in which the booster receives a security query from a mobile communication system and transfers it to the portable telephone, the telephone performs security algorithms and sends an result to the booster and the booster creates an answer to the security query based on the received result. The booster (1) checks the software and hardware versions of the portable telephone (2) and the security procedure is performed based on the version information. Reception of identity information by the booster from the portable telephone also is performed based on said version information. By means of the present invention, equally good security is obtained when using a booster arrangement as when using a portable telephone.