File:GTAIV Russian Revolution Phonecall.jpg

You can call Dimitri once the message "You can phone Dimitri.." comes up, but when the phone connects you also have to be at the correct place for it to work consistently.

The left route shows roughly where the message comes up (when you start dialing) and when you can connect at the earliest. The center route shows when you roughly should start dialing for it to connect safely (if you get around the corner quickly enough), but the message actually comes up at the entrance to the harbor a bit before. The right route doesn't show when it's safe to connect because it seems to be safe even before the message comes up (the location, you still have to wait for the message). This route also includes a drop.

The left route should be the best and a little faster than the center route. The right route could be faster, but the drop makes it a bit dangerous. This assuming that you have parked the car beside the armor facing north (like shown in the map), so you can just start driving north or have to turn around for the other two routes.