The aluminium ring around the phone actually acts as parts of the phone’s radio antenna system. The main antenna is located in the bottom area of the phone and two more towards the top of the phone. There are then stripes that separate the antennas from the other parts of the metal ring.
The antenna technology of the Lumia 925 is able to adjust to the way the phone is held. The main antenna is placed around the bottom corners of the aluminium ring, therefore it is recommended to avoid covering these areas tightly by hand especially in poor network signal areas.