You go to Encore. This place is where you can go talk to your friends and enemies. One of the biggest advantages about having the Visor is that not only are you able to basicallty live in the interweb, but you can unassumingly encroach on someone else without their knowledge. This, explained more clearly, means that you can talk to someone while being in the room their talking to you from. Say, you start cheering Syb, then you would be in his room, talking to him there, and you could feel his face, but he couldn't feel your touch. This continues on to anyone else with a WDG device, which is nearly all devices, so you're pretty set on spying on people.
With this power, you've been able to see and feel all the trolls that have contacted you, these being Karkat and someone else. I could show you Karkat, right now, but that just seems dumb, and would just spoil everything about the story several thousand pages from now, so, unless you specifically request it, Karkat is off the menu.
Now, then, who to call, who to...