The present invention is directed towards compressing and decompressing data transmitted between an end terminal (12) and a network server (20) controlling compression in communication sessions involving the end terminal. States related to the end terminal that are created and used for compressing and decompressing data transmitted between the end terminal and the network server when the end terminal is first registered to the first network server are saved as the end terminal deregisters from the first network server. When the end terminal again registers to the network server these saved states are directly used. In this way it is possible to start compressing data using the saved states immediately after a following registering for providing a more effective dynamic compression.