We 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 top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a right side view thereof.
The broken lines immediately adjacent to the shaded surfaces define the bounds of the claim which extent to the boundaries but does not include the boundaries; the remaining broken lines depict portions of the telephone base unit that form no part of the claim; all broken lines form no part of the claim.