Apple's iPhone success may be reaching its peak

[...] concerns about slowing growth have sent the stock into a months-long slump, fueling debate about what kind of company Apple will be in the future.

When CEO Tim Cook reports Tuesday on Apple's sales for the last three months of 2015, investors will be watching closely for any hints about how Apple's signature smartphone is faring in the current quarter.

Despite some new features, "people are feeling like there hasn't been anything that's really new" in the latest iPhone models, known as the 6S and 6S Plus, which came out last fall, said market researcher John Feland of Argus Insights.

Apple will likely make significant changes in the next major iPhone release, expected in September, which could fuel another surge in sales.

The company went through a similar cycle a few years ago, when iPhone sales growth slowed to 7 percent in the final months of 2013.

While iPhone sales may be slowing, Apple has launched other products and services tied to the iPhone — from the Apple Watch to the digital payments service known as Apple Pay, the subscription-based Apple Music and "smart home" software that lets users control their lights and appliances with Siri, the voice-enabled digital assistant on the iPhone and iPad.

Apple is working on a wide range of future products, from streaming video to virtual reality and even self-driving cars, said FBR Capital Markets' analyst Daniel Ives in a recent note to clients.