Sorry, but the CYAN OS has already been approved Ok'd by Nokia/Microsoft, a long time ago and only T-Mobile is remaining to complete the Apps for their networks. Sadly, it appears they only care about selling the newer phones than keeping current customers happy with a pro-active approach to updates!
It is evident T-Mobile is selling the newer phones with Cyan Update installed and therefore they've already perfected the Apps for it, and they haven't bothered passing on the benefits of that to Nokia 521...!
Only conclusion they are just greedy and will eventually lose a lot of customers to AT&T, as their boss has just announced in the news their service from 2015 will include Wi-Fi calling. So, this means many customers will certainly jump ship to them as that no doubt has kept a lot of people leaving because Wi-Fi calling is a good option where you can overcome no/poor cell phone signal within USA or if like many people travel abroad so that you can make a call on your plan to anyone in USA by avoiding such high cost of international calling home..!
If this fiasco continues for much longer I will jump ship to AT&T very soon, as I can wait for the 2015 date for getting Wi-Fi calling sooner or later!