Abstract: An IMD has its communication circuitry in an inactive state and has access to two different identifier codes. The IMD activates the communication circuitry based on reception of a wakeup message that includes an identifier code that is identical to the identifier code currently assigned to the IMD. A communication device generates and transmits first wakeup messages that include a first identifier code associated with the IMD and second wakeup messages that include a second identifier code associated with the IMD. Depending on the currently assigned identifier code of the IMD the IMD will respond to either first or second wakeup messages.