if you use a UK sim to call a UK sim in Bulgaria your network will take your eyes out, also there is no problems with coverage on any run in pamp and to make it even cheaper just send texts. The bulgarian network also lets you know how much credit you have left every afternoon.
If you have a Virgin phone then Virgin will unlock your phone free.
I paid £10 to unlock my phone the 1st time by the man on the market. When i needed a replacement i mentioned this to Virgin and as long as i had spent £30 on credit then they would send me the codes in the post.