Apple users have been reporting an issue with their $1,000 iPhone X models that is stopping them from answering calls.

The issue was first highlighted by the Financial Times on Sunday which pointed towards Apple support forums.

One user described how when they received an incoming call on the iPhone X, the ringtone began playing but the display did not turn on until after six-to-eight seconds. The user said that the problem gets solved temporarily when they restart the phone, but then comes back after about 15 to 20 calls.

Another user posted on the discussion thread on Jan 31 describing a similar problem. The user said that they updated to iOS 11.2 and then to the latest version of Apple's mobile operating system, iOS 11.2.5. The problem still persisted however. The customer claims they have been dealing with the problem for 30 days.

Apple told CNBC that it was "looking into these reports."

Market research firm Canalys estimates that Apple shipped 29 million iPhone X units in the December quarter. In the quarter, Apple sold 77.3 million iPhones, which was fewer than the same time a year ago.

The iPhone X is seen as a radical departure from previous models with new features such as Face ID which lets users unlock the device by looking at it. But it has been hit with issues such as delays and reports that Apple has cut production of the premium handset.

Apple shares have been under pressure in recent days. On Friday, Apple shares closed in so-called correction territory.

Investors have also raised questions about the iPhone X in the long run. Apple issued revenue guidance for its fiscal second quarter of $60 billion, which was below what analysts were expecting.

Some users have complained about Apple's software updates of late, and news site Axios reported that the company would be delaying some new iOS software features, to focus on performance and quality.