I made several calls home from my hotel room (Orlovets in Pamporovo) - and was amazed how little I was charged. Although not itemised it would seem to be around 1 lev per minute. I was charged under 15 levs, and must have talked for the best part of 20 minutes.
In Bansko last year (I know you said Borovets but they might have the same thing) there was a building with two phoneboxes in and a counter. You went in and used the phone, then when you were finished you went to the counter and paid, I was on the phone for about half an hour and the charge was next to nothing. I can't remember what the building was called but it was by the Pirin hotel.
I was in Pamps over New Year, £1.40 a min & 40p a text, It's £1.40 a min when people phone you too so best to leave your phone off. I drunkenly phoned a mate New Zealand on the 2nd, wasn't the cheapest phone call I've ever made!
Buy a pnone card from a shop or hotel reception either the blue or orange card depending on the phone you want to use. Cheapest way I know of for phoning home and when its done its done! To phone others in Bulgaria buy a couple of Bulgarian sim cards!