These are the 3 major phone companies in Bulgaria:
SIM cards are often free with an initial purchase of credit. The cheapest phone credit will cost you 6 leva (€3). You don’t need to sign a contract or pay monthly bills. You can charge your prepaid SIM card whenever you need. You need to provide your full name, ID card number and personal identity number to obtain a SIM card.
Every prepaid SIM card should be registered. Please note that mobile operators cannot register and/or activate more than 10 prepaid numbers under one name.
You can find many special deals where you have free internet data, free call minutes and free text messages. This can help you stay in touch with family members and friends in your home country.
You can purchase phone credit/vouchers at kiosks, shops and official providers' offices.
You can find public phone booths in most towns and villages, most likely in post offices, but they are usually broken. If you find one that works, you will need to buy a special phone card to use it. But, because most public phones are broken, phone cards for them are hard to find.
It might be a better option to go to the central post office, where you can make calls on a public phone and then pay the cashier directly. This arrangement is convenient for long-distance calls.