I claim the ornamental design for a telephone base unit, as shown and described.
FIG. 1 is a front perspective view of a telephone base unit of the present invention;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a first side view thereof;
FIG. 5 is a second side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines depict portions of the telephone base unit in which the design is embodied that form no part of the claimed design. Additionally, the dot-dash lines denote the boundary of the claim and form no part of the claimed design.