To listen to the first conversation, as soon as the mission starts give your man the order to stay in that position so he doesn't get in the way and then run down and hide to the side of the big construction vehicle to listen to the conversation. Then wait for Atongwe and his followers to walk away and walk up the stairs as soon as there out of sight ignore the guards and run across and follow them, as they proceeded to walk through the building and walked down the small steps, stay at the doorway and listen out for the second conversation.
As they start walking away, slowly follow them to the other side of the compound and hide behind the blueish vehicle to the right hand side, you should hear the third conversation. Watch Atongwe and his followers walk inside the building and wait the the guard near the door to walk away, swiftly move into the building so the guard doesn't notice you. Wait for them to walk up the stairs and slowly follow them, as the door closes at the top wait a few seconds and then walk towards the door and just peak around the corner to listen to the fourth conversation, as Atongwe walks away the achievement should unlock.