Every US carrier has a threshold for customers on unlimited data plans. Usually, the threshold is around 22GB within a single billing period. What this means is once you hit the threshold, your data session will be set to a lower priority than others who are on tired-data plans, or to those who haven't yet reached the threshold. T-Mobile's deprioritization threshold has been 28GB in the fine print of its unlimited data plans for some time now. T-Mobile has quietly updated this threshold to 30GB within a single billing cycle. This section of text can be found in the company's "About" page...
Source: http://www.gsmarena.com/tmobiles_throttling_threshold_raised_to_30gb_per_billing_cycle-news-23862.php
