Unrealistic! If you're in a coffin, you wouldn't have enough oxygen to burn the lighter and still breath for hours. If you know that you wouldn't light it up while you talking on the phone unless the director wants you to leave it on so people can see your face. If you just need to grab a little bag close to your feet in a narrow coffin, you would just use your foot to kick it up toward you. If you told the guys who were searching for you about the jet just flew over you and bombed, they would quickly locate your location in 1 minute. If the sand keeps pouring in and start to burry things, you wouldn't float to the top of the coffin like you are in the water with a floatation device.
If I knew it was like this, I wouldn't start watching it in the first place, even though it has a very interesting way of making the movie with only 1 character for the entire movie.