State-run MTNL slashed ISD calling rates to six countries, including US, Canada and China, for its GSM and 3G subscribers to 1 rupee a minute. Brought about to mark MTNL's 25 years in service; the call rates will be applicable for a period of three months.MTNL CMD Kuldip Goyal told newspersons in New Delhi that the call rates to Hong Kong, Thailand and Singapore will also cost 1 rupee a minute. The PSU has also cut the call rates for Australia, Austria, Bangladesh, Columbia, Indonesia, Iraq, Russia, Taiwan, Uzbekistan and Vietnam to 5 rupees per minute. The tariff will come into effect from today.