Better still Paul, make sure you take a couple of unblocked phones with you and buy two bulgarian sim cards with some pay as you go credit, when you get there. This will be a hell of a lot cheaper for you. There's a phone shop in The Samokov hotel who can sort that out.
There may not be a probem using your phones but the charges will be very high. If possible get WT or buy a bulgarian sim card, which I'm sure someone on here will be able to help you with.