Siemens Gigaset Premium E390HX: Big Buttons, Extra Loud, Crystal Clear Conversations. Please note that this is an additional handset and charging cradle to be purchased for use with an existing system. This handset cannot be plugged into a telephone line, and will not work without a base station (The Gigaset E390A main unit with base is available here).. Up to 6 handsets can registered to the base of the E390A. Benefits of using multiple handsets include features such as: intercom, call transfer, and conferencing facilities.. As this handset is GAP compatible, it can be registered to other Gigaset bases. For details on what features will work when using the E390HX with a different model, please refer to the following compatibility guide.