in case of BSNL GSM prepaid if you activate any tariff voucher from main balance they dont charge additional service tax, when I was on bsnl Rs.17 voucher used to cost me around 15 as I already paid taxes while recharging account. May be similar case be with postpaid,they charge taxes on rental only.
Also I have used EVDO with automatic rental deduction plan at that time it used to deduct Rs750 from main balance while voucher used to cost 800+ that time.