(a) In general. The taxes imposed by section 4251 do not apply to any amount paid as is properly attributable to the installation of any instrument, wire, pole, switchboard, apparatus, or equipment.

(b) Maintenance charges subject to tax. The exemption provided by section 4253(g) and paragraph (a) of this section is applicable only to amounts paid for installation. Amounts paid for the repair or replacement of instruments, wires, poles, switchboards, apparatus, or equipment, incidental to ordinary maintenance, are subject to tax.