Updated with comments from Jim Cramer.

NEW YORK (TheStreet) -- It's no secret that Apple (AAPL) - Get Report gets more than half of its revenue from the iPhone or that whenever Apple releases a new model, fans come out in droves for the new device.

But as soon as anticipation starts building about the next model, sales of the current version generally start to slow.

Except, that isn't happening with the iPhone 6 and 6 Plus.

Citigroup analyst Jim Suva wrote that iPhone demand is still "healthy globally given solid underlying build and sell through," upping his estimates for the fiscal third quarter to 49 million units, above the 46 million estimate comprised of several Wall Street analysts.

Though there are no official estimates, analysts polled by Thomson Reuters expect the Cupertino, Calif.-based Apple to earn $1.76 a share on $48.6 billion in revenue for the fiscal third quarter.

In the fiscal second-quarter, Apple sold 61.2 million iPhones, generating $58 billion in sales for the quarter, up 27% from the same quarter the previous year.

"I know Apple acts horribly but I think that what matters is iPhone sales have held up well and I continue to believe the right course of action is to own Apple not trade it," said TheStreet's Jim Cramer, Portfolio Manager of the Action Alerts PLUS Charitable Trust Portfolio.

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Much of the strength of the iPhone appears to be buyers enticed by the larger screen sizes, as Apple introduced a 4.7-inch and 5.5-inch iPhone in Sept. 2014 and not from existing iPhone users, indicating there's more strength to come.

"Our current estimate is that about 20% of the active installed base has upgraded to a 6 or 6 Plus," CEO Tim Cook said on an April 27 earnings call. "And so as I look at that number, that suggests there's plenty of upgrade headroom."

The iPhone has become a global juggernaut, taking market share in almost every country, especially in China, where Cook noted it took more than "9 points of share on a year over year basis." According to research firm IDC, the average price of a smartphone in China rose drastically, rising to $263 in the calendar first quarter of 2015, up from $192 in the third quarter of 2014.

The question remains whether it can stay that way, now that Apple has already enlarged the iPhone, one of the key drawbacks against it and a primary selling point for competitors like Samsung (SSNLF) and other Android-based devices.

With China's economy slowing (it recently cut interest rates for the fourth time since November), it's unclear whether the iPhone can continue to be the juggernaut it has been globally, as well as domestically.

Apple is expected to unveil a new iPhone later this year, perhaps as soon as September, with several new features. In addition to a better camera, a new version of iOS and several other new features, Apple is likely to bring Force Touch to the iPhone according to a report from Bloomberg, a feature that brings up different functions, depending upon how hard the user presses the screen. Currently, the Apple Watch and the recently announced MacBook offer Force Touch.