Open the file in CorelDRAW 8 or above. When you open the file, it should be properly positioned on the page for an iPhone to be engraved. To be sure, mask the back of the phone and run the file at a low power and high speed so it will only engrave the very top of the masking material. This will allow you to make sure that the engraving won't run over the camera lens or the Apple logo.
Run the laser in engraving mode for this test - around 100% speed, 15% power on a 35 watt system.
When you are sure that the image is perfectly positioned, send the project to the laser with settings you would use for an anodized aluminum material.