The WORST telephonecompany in the WORLD - I think it is.

Mtel in Bulgaria - Mobiltel - charged me wrongly for approx. 1000 leva (500 Euros) in a periode of about 18 months. That was in 2008 to march 2010. The mistake was, their system did not register it correctly when I called from Bulgaria and to Europa. I had 100 minutes for free for stationary phones in conties in Europe - but I paid for these 100 minutes - I found out. One mistake was it registrated it like I called cell-phones in Europe - which costed about 1,5 leva per minut (0,8 euros per minut). The other mistake was, that it registrated it correctly but did not deduct the 100 minutes included in my package.
By mistake I never saw that (but just paid the phonebill) untill 1½ years later. Mtel repaid me about 400 leva (200 euros) for the last 6 months - but the rest 12 months (500 euros) they would not repay - for no reason. So their mistake (they even admited it) - and they do not pay it back. This is the worst I have experienced with telephone companies ever. What the h. can I do about it ? It is 500 Euros. I can only say - f. Mtel.