It may be an awesome phone, as evidenced by its topping the charts on Amazon, but the Nokia Lumia 900 isn't without its share of problems. According to Lumia 900 owners on both WPCentral and Nokia's own forums, the flagship Windows Phone from Nokia will lose all data connectivity if the cellular connection is interrupted, even by something as simple as airplane mode.
Only a temporary fix has been found, and is fairly complicated. Affected users need to take out their phone's SIM, boot the device without it, shut down and put the SIM back in, and return to normal use. However, this solution is only temporary, and the problem will return after a short time.
Speculation is that a problem with some device's IMEI numbers (a smartphone's unique identification to the network) weren't registering correctly on AT&T's data network. A source has it that AT&T is taking problem devices off the shelf, but the carrier has yet to confirm any of the problems.
Severity is still unknown, but the problem doesn't seem too widespread. Still, buyers should be aware of the issue, if only so they can go to AT&T and get a warranty replacement. The problem doesn't detract from how good the Lumia 900 is, but only serves to illustrate that nearly all devices will have problems, and some unlucky users may have to go through the hassle of exchanging their device.[Electronista]