Sim card of Montenegrin phone

We got used to low roaming costs in European Union so we always take our mobiles with us. Some people think in Montenegro is the same... because it's Europe. Even first customer service representative from my polish mobile operator tried to convince me that i can buy great discounted packets to call and with internet in "Eurozone". When i said Montenegro is not in European Union, she had to verify few times and then she had to apologize: there are no special call tariffs to Montenegro.