Microsoft planned to announce on Tuesday that it will start accepting applications for its Windows Marketplace for Mobile on July 27 and confirmed that not all existing applications will be available at the store.
It also said that the store will be open by the end of the year to users of phones running Windows Mobile versions 6.0 and 6.1.
Microsoft had previously said that the Marketplace would be available only on phones running Windows Mobile 6.5, which is due later this year.
While Microsoft often says it has 20,000 applications available for Windows Mobile users, not all of those applications will appear in the store immediately.
The Marketplace won't aggregate all available applications, only those that Microsoft has tested and guarantees with a money-back offer.
Some recent reports have suggested that around 600 applications will be available at launch.
While a company spokesman confirmed that number has been given as a preliminary estimate, he said it's too early to know how many apps will be in the store at launch, since developers haven't yet begun to submit their programs.
Microsoft is playing catch-up to Apple, which popularized the concept of downloading phone applications with its iPhone and iTunes applications store.
Apple recently said that 1 billion iPhone applications have been downloaded. Others have already followed Apple's lead, with application stores available for Android and BlackBerry devices, as well as the new Palm Pre.
Microsoft is competing for developers with those phone makers, although many may build applications for multiple platforms.
While developers may be attracted by the millions of Windows Mobile devices in use around the globe, those phones have considerably less buzz compared to the iPhone and other popular phones.