.. Where can I use my cell phone?

As always, Nancy is a courteous phone user; she won't talk on her phone when anyone else is within hearing range. In addition, she can't get coverage inside the tomb, which makes sense. That means she can make calls from the antiquities tent, next to the door in the residence tent, and out in the open. After Dylan shows up and parks himself next to the water truck, you're restricted to the tents and the steps leading into the tomb.