Nowadays, cellphones are all about apps. And Microsoft is so determined to have lots of brand-name apps for its Windows Phone app store that it is willing to pay for them.

All an app maker has to do is sign on the dotted line.

After years of struggling in the phone market, Microsoft teamed up with Nokia last year to challenge the dominance of Apple’s iPhone and Google, which makes the Android operating system. The latest fruit of their collaboration is a gleaming machine called the Lumia 900, which goes on sale in the United States on Sunday and is considered to be the first true test of how well the partnership will fare.

But the hundreds of thousands of apps that run on Apple and Android devices will not work on phones like the Lumia 900 that use Microsoft’s Windows Phone software. And many developers are reluctant to funnel time and money into an app for what is still a small and unproved market. So Microsoft has come up with incentives, like plying developers with free phones and the promise of prime spots in its app store and in Windows Phone advertising.

It is even going so far as to finance the development of Windows Phone versions of well-known apps — something that app makers estimate would otherwise cost them anywhere from $60,000 to $600,000, depending on the complexity of the app. The tactic underscores the strong positions of Google and Apple, neither of which have to pay developers to make apps.