A mobile phone falling from a feet or two hardly causes skull injuries but again, it depends on the angle and the surface area that you are hit on. A dent may be nothing, or it could be a depressed fracture of skull( considering it is persistent for more than a day). Also, better to get a CT Head plain done if there really is a dent and you have persistent headache, it will help to clear the doubts as well as guide further treatment.
Next Steps
You can consult me on practo VC with or without a CT head.