An aircraft may have a mode S transponder but in order for the callsign to show on a radar screen (without any further actions by controller or pilot) the ground equipment has to be suitably equipped. Unfortunately, it is not easy to know how things are set up on the ground unless one actually works with the equipment and has an opportunity to learn how everything is connected up. It's quite rare to find a stand-alone radar - most radar heads are connected to a 'chain' in which all information gathered by the head is put into a pool along with all the other sources in the chain, and the best information available for a particular area to be displayed on screen is used. So, for example, if one radar head is taken out of service for maintenance or whatever, or another head is added to the chain, the procedures that must be used by a controller may change, either temporarily or more permanently.