As I was finishing the quest 'Pact with Rais' and going back to Rais' hideout, I heard a person calling me and saying he's on top of the antenna. He also asks if I can see him. Would like to know what side quest is this and where is he? I heard his voice as I was running along the pipes in the area marked below. This is the area which seems to always have a Toad or two and looks like a factory.
I can't seem to find him and the nearest antennas seems to be so far away that someone shouting from there couldn't possibly be heard.
Best Answer
After going through the list of side quests found on game pressure I believe I have found the sidequest.
I believe this to be the Prodigal Son sidequest as it becomes available to you when completing 'The Pact With Rais', as well as the fact that Alexai (the one you need to speak to to trigger the quest) is found on an antenna tower and from this video the location seems to be quite near to your screenshot.
He is located at antenna 6a on the map. (apparantly)
Side note: I do not play the game so this answer is based on what I could find out online.