Phone companies are concerned after hundreds of mysterious calls on landlines to Kiwis across the country in the middle of the night.

Hundreds of people commented on a Dunedin social media page this week about being called by private numbers, mostly between midnight and 4am on Tuesday.

Spark New Zealand communications adviser Lydia Tebbutt said its investigations team was aware of thousands of similar calls across the country in the past few days.

The caller would phone landline customers from a hidden number and hang up once it was answered, she said.

The offshore number had been identified and blocked.

An Auckland Vodafone customer said she woke up to the sound of her phone ringing "in the middle of the night".

"I thought 'Oh gosh, who's died?' No one calls me on my landline at this time of night.

"It stopped ringing before I got out of bed to answer it. That made me think, 'So who was it?' It was incredibly annoying."

Spark did not have information regarding these calls, but was concerned about the risk of a scam, Tebbutt said.

''Spark would like to assure customers that there has been no security breach, and we would encourage any customers who receive calls of this nature not to phone the number back.''

Vodafone spokeswoman Alice Paine said it did not receive complaints about this particular instance, but saw this type of scam before Christmas.

''These types of scams usually present as international numbers. However, it can also happen when an international call presents on the phone as a local New Zealand number - a tactic known as caller ID spoofing.''

It was hard to track the calls, but they were blocked when they could be, she said.