If you’re looking to buy a new Nokia Lumia 900 Windows Phone this week, it might not be as easy as you’d expect.

AT&T’s online store is showing the Nokia Lumia 900 as temporarily out of stock in both colors, and Amazon.com’s wireless store shows the cyan version backordered.

Situations like this are tough to judge, because it’s hard to tell whether they’re caused more by short supply or excessive demand. But last week an AT&T executive told the Wall Street Journal that initial sales of the Lumia 900 had exceeded the company’s expectations, as noted by the Verge.

As long as the supply can be quickly restored, it appears to be a good sign for Microsoft’s attempt to come back in the mobile phone business. The company is counting on the Lumia 900 to start to boost its market share in the U.S. against the iPhone and Android devices.

The Lumia 900 launch hasn’t been entirely smooth. Nokia last week issued a software update and promised a $100 credit to address a problem with the phone that was causing some users to lose their data connections. The company also offered replacement devices as an alternative to the software update. The problem was related to the device’s firmware, not to Windows Phone.

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