Apple is said to be investigating reports of the iPhone X not displaying the caller ID or Accept and Decline buttons when someone is calling.
The Financial Times reported that Apple has been looking into complaints that have been reported in relatively small numbers over the past three months.
The first news report of the issue came in December from Indian blog PiunikaWeb, which linked to several threads on Apple's forums detailing complaints dating back to November.
The first person to report the issue on Apple's forum claimed to be using an iPhone 6 with iOS 11.0.3 and posted the report in October. However, subsequent reports refer to the same call-display delay issue affecting the iPhone X.
There are also a few reports of the issue on Macrumors' forum, with delays of up to 10 seconds resulting in users missing calls.
"Whenever I receive an incoming call in my iPhone X, ringtone starts but the display comes in after six to eight seconds. When ever I restart my iPhone X, the problem gets automatically solved and after 15 to 20 calls the same problem starts again," one user reported.
As Macrumors notes, there was also an issue where the rear-facing camera failed to autofocus, and green lines appearing on some displays.
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Attention on the latest apparent issue comes after last week's report that Apple is delaying a number of new features in iOS 12 to focus on performance improvements. The delayed features included a redesigned homescreen, CarPlay updates, and improvements to the Mail app camera app.
Apple last week also offered to replace a small percentage of iPhone 7 models with a failed component on the main logic board, which resulted in devices showing 'No Service' in the status bar even when cellular coverage is available.
Previous and related coverage
It shouldn't come as a surprise to anyone that a device as ambitious as the iPhone X suffers some teething troubles out of the gate.
Burn-in is a fact of life for OLED displays, including the iPhone X's Super Retina display.
Rapid drop in temperature causes some iPhone X screens to become unresponsive.
These iPhones could hit their 500 recharge cycle limits much sooner than expected.
The earnings report answers a lot but doesn't settle the question about how the iPhone X is selling.