DCS is a secondary GSM standard used for voice signal transfer to your mobile phone. It works on high-range 1800 MHz frequency available predominantly through the cities.
Most mobile network operators use DCS 1800 as additional to GSM 900, that’s why it’s also known as GSM 1800. Still some carriers in certain countries provide voice service on DCS 1800 MHz only (e.g. Orange in Luxembourg, Yoigo in Spain, etc.).