A caller associates computer software code with a signalling protocol messages such that when the messages are received at a destination processor the computer software code is executed. For example, the messages may be improved SIP protocol messages with incorporated Java code. By selecting different computer software code for association with the messages, the caller is able to control the destination terminal. For example, to display information about the identity of the caller at the destination terminal; to modify the behaviour of the destination terminal according to the priority of the call; to take into account the configuration of the destination terminal, and to allow users to adjust this configuration from a remote location.