Next, Microsoft’s (MSFT) Windows Phone OS will conquer the iOS as Nokia (NOK), which is phasing out its Symbian OS by 2016, adopts it.

IDC is predicting that Microsoft’s markets share will reach 20.3 percent in 2015 while Apple will be at 16.9 percent.

IDC said Android will be the leader with 43.8 percent in 2015 a year when one billion smartphones are expected to ship. That’s about double the number of smartphones expected to ship this year.

The forecasts, which are being questioned, appear in IDC’s Worldwide Quarterly Mobile Phone Tracker report. Gartner has made similar rosy predictions for Microsoft. And IDC has revised numbers from earlier this year.

How will things look later this year?

PCWorld reported: “At the end of this year, Android is projected to have the most market share at 38.9 percent, IDC says, followed by Nokia’s Symbian at 20.6 percent, and then iOS at 18.2 percent. However, Nokia has declared Symbian dead and its current market position relates largely to Nokia’s traditionally dominant position in global markets, particularly those outside the U.S., IDC noted.”

Nokia experienced declining sales when it announced its plans. And the Nokia phones on WP aren’t expected to hit the market until later this year.

Keir Thomas reported in PCWorld that IDC is betting that Nokia’s adoption of Microsoft’s OS will be easy.

But there is plenty of skepticism about a smooth transition. Stephen Patel, an analyst with Gleacher & Company, said, “Our checks suggest mixed carrier support for Nokia’s transition to WP.”

Ryan Kim said in GigaOm: “(T)he latest predictions show how perilous it can be to make firm forecasts in such a fast-moving market. So much of the WP7 prediction is predicated on a smooth transition for Nokia. But if anything, the latest news out of Nokia suggests that it’s anyone’s guess how well that will turn out.

Android cutting into Symbian already

Nicholas Kolakowski noted in eWeek: “Android sales are supposedly eating into market share once occupied by Symbian.”

Also, IDC thinks that the Mango update of Windows Phone 7 will give Microsoft a boost.

Kim said: “Windows Phone 7 is a solid operating system and will get even better with the Mango software update this fall. But it’s not enough to just have a good OS. WebOS was solid, too. You need a lot of other things to fall into place, like great carrier support. Yet, some are questioning operator support for WP7 right now. And with Android and iOS building huge ecosystems, challengers need to find ways to leap ahead.”

International Business Times said: “(T)here is inherent conflict of interest between analyst firms like Gartner and IDC, since the majority of the companies they write about also are clients. IDC has done some mighty Microsoft-friendly research or white papers over the years. But this business about Windows Phone is just too much to believe. IDC’s forecast validates Microsoft’s Nokia-Windows Phone distribution deal without even one handset shipped. It’s sheer speculation in a market changing so fast — Gartner and IDC are forced to revise their forecasts every couple of months (without really acknowledging how wrong they were so short a time earlier).”