I claim the ornamental design for a cell phone, as shown and described.
FIG. 1 is a left side elevation view of the cell phone showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof; and,
FIG. 4 is a right side elevation view thereof.
The broken line showing of environmental structures on the front of the cell phone in FIG. 2 is for illustrative purposes only and forms no part of the claimed design.