TL; DR – http://m.mtel.bg/surf
If you have a mobile data plan from Mtel (USB modem or integrated one) and you use Linux, you can’t really check your data usage because the software Mtel provides is Windows-only. You can always use a watchdog of some sort that monitors the traffic on the client side but that’s not really accurate and is kind of cumbersome.
Or you can use this. It just shows your remaining/used mobile data. Since it is kind of designed for smartphone users, there is only mobile version (hence the ‘m’ in the URL). Also, you can ONLY access the page from Mtel’s network (that same sim card you want to check up on).