The Cell Phone will be used in the Giza area where you will have to dial 555-6789 to distract Vince Grave and go to his tent, this will let you sneak up to his bag and get the Moonstone, which is contained inside.
How to Get the Cell Phone
When you reach the top of the Mountains of the Moon, you’ll see an old man above you but you can’t jump directly to him. The old man will challenge you to a game of Mancala. Once you begin the game, you’ll learn the rules if you don’t already know them. When you win, you can enter the cave behind him. Drop down the left side to get to the bottom, where you’ll find an old cell phone.