The swindlers, who claimed to represent the NBTC, would ask for a copy of people's IDs and promised to deliver the set-top box to them.
The NBTC has never appointed any company to do so, said secretary-general Thakoon Tanthasith on Saturday.
The regulator is still considering the pricing of the coupon, he said.
It plans to start sending the coupons to 22 million households by mail in July, starting with the areas where the signals are ready.
The coupons can be used to buy at a discount a set-top box plus an antennae, or a new digital TV set.
For those who want to buy a new TV set now, he recommends buying those with the NBTC halogram Class A Broadcast mark and its Doodee sticker.
NBTC commissioner Supinya Klangnarong said the regulators are debating whether the coupon should be worth more or less than 1,000 baht.
They are also undecided whether the coupon could be used to buy a satellite TV set-top box as the equipment can also carry the digital signal.
Thailand started a digital broadcast trial on April 1.
The trial covers Bangkok, Chiang Mai, Nakhon Ratchasima and Songkhla.
The operators of free TVs — Channels 3, 5, 7, 9, 11 and Thai TPBS — are muticasting on both the analogue and digital platforms.
The coverage will be expanded to cover 80% of the country by mid-2015.
The NBTC will issue a coupon for each household to facilitate the transition.