I used to deal with glass and glass cutting. I really believe it would be better to cut through the glass and then to try finishing the operation on the CNC.
The best way that we found to cut any glass was to take a piece of copper pipe and to cut a notch on one end that encompassed less than ⅛ of the entire circumference. The notch should be in the form of a right triangle with the vertical face at the rear of the cutting direction. This allows for water and carborundum to flow in, in front of the vertical cutting face. Some people have used this method, omitting the carborundum, but I found the cutting to be much slower, and more prone to cracking. The “water only” method does leave a smoother finish to the cut, however.
Build a dam of plasticine clay at least 20% larger than the hole that is to be cut. Place the copper tube into a drill press, add water and carborundum grit to the reservoir that was created and, using gentle pressure and a slow drill speed, cut your hole. Make sure to keep the reservoir full of water at all times.
You will get water and grinding grit as well as glass slurry into your phone…but, it sounds like keeping the phone working is not important. The water allows for cutting without the vibrations that cause glass to shatter (try cutting a pane of glass, underwater, with ordinary scissors. It will blow your mind).