They are available after you complete the last tutorial mission. After completing Delivery, an orange triangle pops up next to all the antenna icons on the map. When the player goes to any cell tower, there will be a "call box" on the side of it. When interacted with, the player character will receive a phone call from a shady contractor using a voice changer who will instruct him to assassinate a "target of opportunity" who is located in a certain area of the map. After the target is killed, the payment will be given instantly.