A fixed or Mode II device enables a Mode I device for license exempt operation by providing to it a list of available license exempt radio channels and a parameter indicating an interval at which contact verification signals CVSs are to be sent. The CVSs verify whether the list remains valid. The fixed/Mode II device then repeatedly sends contact verification signals spaced in time from one another according to the interval. In various embodiments the parameter may be an explicit indication, or it may be implicit. A plurality of enabled Mode I devices each gets a device-specific list, and in one embodiment the interval is the same for each of them and the explicit indication is broadcast while in another the parameter is provided to each of them via unicast messages and the interval may differ.