Roaming in Poland - data works, but no calling

I have contacted support, and they have solved the problem. To all that may experience a similar issue: it was the network operator. Smarty supports a subset of operators in each country, and my wife's phone was automatically choosing the one that wasn't supported. What was needed was to disable automatic operator selection, and choosing manually one that is supported by Smarty in Poland. That's it. 

Unfortunately, Android does not have an option to disable specific operators, so we will have to change that option back and forth when travelling between countries.