Hi, yes they do monitor the alarms. We set it off by accident in our apartment in Sofia one day and there was an armed cot guy there within three minutes! Can't remember how much we pay, but it's a monthly fee.
You'll find contact details on their web site
СОТ - Сигнално Охранителна Техника - the site is in English as well as Bulgarian
All the best, Jain
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Jain Goodall, Director, Quest Bulgaria Magazine
Independent advice and information for owning and buying property in Bulgaria
http://www.questbulgaria.com