You wont use any! It will be Mtel or global. Make sure your phone is set for international roaming. It is advised to get your phone unlocked before you go and buy a sim card whilst there to contact each other for cheaper calls.
It is better to buy a sim card while you are here. As above, mtel, global and vivatel. My network is vivatel but i am changing to globul as with vivatel i cannot text the uk. Can ring, but not text. To buy a sim card is about 7 leva then just put a 10 or 20 lev voucher on. If your fone is unlocked, you can just swap the sim card for the time you are here. Much cheaper than using your uk fone here.