The ornamental design for a phone case, as shown and described.
FIG. 1 is an isometric view of a phone case showing my new design.
FIG. 2 is a front view thereof.
FIG. 3 is a top view thereof.
FIG. 4 is a left side view thereof.
FIG. 5 is a right side view thereof.
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a back view thereof.
The broken line showing of the rectangular-shaped feature on the rear of the phone case depicts environment and forms no part of the claimed design.