Anyone who has a Congstar postpaid contract and wants to know how much volume remains until the compulsory throttling, can obtain the relevant information about pass.telekom.de. The page only works over the T-Mobile network and not in the WLAN, the assignment happens automatically. It shows the data consumption in the current billing period, in addition to the volume already consumed, it also shows the remaining time until the reset. Besides the speed managerwith which various compression settings can be influenced, there is another service that you already know from the parent company - and even in the Congstar look.
Although the data is available with a slight delay (~24 hours), you get the data volume that Congstar actually uses for the calculation of the throttling - with apps on mobile phones it can happen that they measure too much or too little traffic and therefore there are deviations in the data volume.
Update 2014-03-30: Problems with Google Chrome
If you use the option "Reduce data usage" in current Chrome versions, you will not be able to access the Telekom services. You will be warned that the page is not available. This is due to the fact that Google uses a proxy for data traffic when this option is activated, so the Telekom servers can no longer tell which mobile phone it is.
If, on the other hand, the option is deactivated ("Settings" -> "Bandwidth management" -> "Reduce data usage"), it is possible to query both the data volume consumed and the Speed manager without any problems.