Consumers have been warned about a phone scam that encourages them to call back an unknown number at a huge cost.

Speaking on RTÉ's Morning Ireland, the Director of Retail and Consumer Affairs at ComReg said a substantial number of consumers have had missed calls from foreign numbers over the last couple of days.

Barbara Delaney said that generally the call does not ring long enough to be answered.

This is because the point is to heighten peoples' interest to call back the missed number, which is a premium international phone number.

"The scam is really to trick you into calling back and when you do call back, you may be enticed into listening for longer and the longer you stay on, the more it'll cost you," she said.

Ms Delaney warned consumers not to call the number back.

Often, she said, people returned the calls because they had friends or family members living abroad.

"I know people may have relatives or friends abroad and they're concerned that maybe somebody they know is calling them," she said. 

"I would advise them to be familiar with the country code of the people you think might be calling you. France is +33, the UK is +44 and so on. So be familiar with that, store their number in your phone."