Where do you go to call the phone number on nabooti island?

You need the cell phone from the Mountains of the Moon. Climb up and beat the old man at mancala to enter the cave.

You use the phone at Giza when you get the shovel from the tomb raiders. The number on the shovel is 555-6789. When you are brought to Vince (the head man) call the number to distract him. You can recover the Moonstone from his bag while he is looking for his phone.

Put the Moonstone in the ring above the Sphinx's head to open the tomb.