This is just a DECT handset without a base station and you need a compatible base station, e.g. swisscom Internetbox or Gigaset base station

Gigaset CL660HX
RJ11 connector is top,
A good design, and various functions.

And there's the ECO DEC mode
This mode reduces energy consumption by up to 60% and radiation by up to 80% compared to phones without ECO DECT.

You don't necessarily need to connect it to the Gigaset base station
thanks to connection options to all compatible internet boxes.