Passu is one of the last towns on the Karakoram Highway-. It has no internet access and not all mobile networks have a good signal there. However, there's no police around and the Chinese camp of road workers is convenient enough for hitchhikers. Go there at any time of the day (they start at 05:00) and ask if they can drive you to Sost - they'll be happy to help.
If going the opposite way, you can only get a ride to Gulmit or Hussaini from where you must take a boat across a veeeeery long mountain lake. There is a free tunnel built. The lake is very panoramic but not necessary and you have to pay a fee. November 2016
You can find Borit Lake between Pasu and Hussaini. Great campsite away from the KKH. I camped four nights for free with permission of the hotel owner.