Calling the shot is knowing, with a fair degree of certainty, who you are going to shoot. Military intelligence is never 100%, and shot calling is similiar. You track the player by sound and patterns, move into a spot where they are most likely to cross your path, and then when you see that person, you shoot. Watch for the different player faces people use, and also different uniforms and weapons. If you are hunting a specific person, it is better to let another opponent go past your position without shooting. That way they may think you are not in that area, and they can safely move past your spot.
A personal story: One time went into a TKOTH and saw a large group of team players stacking one side. Our team had two people on one side and one on the other side, and we weren't using any voice programs. Before the game started, I informed the leader of their team that they were going to be targeted and hunted by me. As soone as the game started, I ran an outflank and took on their left flanks, and took them out. Once I managed to control the one flank, started waiting in ambush spots in center, to take out their team players. Then went on to do a QRF and stop any team player that got hot. In a 25 on 25 game, where they had 10 team players and 15 random players on one side, against 25 random players, we managed to win that game, because the 10 team players were effectively targeted and nuetralized. When the game was over, 85% of my scores had been on the team that was being targeted, and the other 15% were people who were in the wrong spot shooting at me. I called almost every name before shooting. It was a good feeling to watch the people who were "sucking up" to the other team leader start doing it to me when they saw them get nuetralized so well.