Prepaid Lumia 521 sold out on the same day of becoming available at T-Mobile 3 days back. But it was back in stock soon and seems the second stock has lasted for 2-3 days this time, as it has again sold out. As you can see in the above screenshot it is out of stock for the second time in row at the carrier website. If nothing else this proves good demand for the device as we have seen similar demand at both HSN and Walmart.
Thanks “T” for the tip. Cheers!!