Essentially the same phone, the low price of the two models have helped them ring up the cash register. During the recent conference call following Nokia's second quarter earnings release, CEO Stephen Elop mentioned the strength of Nokia Lumia 520 sales in places like China, France, India, Thailand, the UK, the US and Vietnam. T-Mobile's version of the phone, the Nokia Lumia 521, has been sold out at a number of locations and has been burning up the phones at HSN where the phone has made a few appearances.
The Finnish OEM has made it clear that "hardware restrictions" prevent the Lumia 520 and Lumia 521 from offering the Glance Screen. Perhaps with the money you saved from buying either one of those two models, you can invest in a cheap watch. Meanwhile, the screen is already installed on the Nokia Lumia 925, which launched with the GDR 2 update out of the box. Besides the clock, the Glance screen allows you to double-tap the display to wake up the phone, and includes certain notifications.
Nokia reveals that there will be no Glance screen for the low-end Nokia Lumia 520 and Nokia Lumia 521 models
source: Nokia via WindowsPhoneCentral