Poland’s country code is +48

Poland is actually quite cheap when it comes to data. If you’re looking for a brutally honest opinion on what carrier is the best, then it would have to be Orange.  

They offer 50GB for 5 months each time you top up 100PLN via their website.

Also, Polish law requires everyone to register their number. This can be done in any carrier store, and ‘newsagents’ like InMedio which can be found beside the dormitories. You simply take your passport along with you, provide them with your number and thats all. 

If you are from the EU, you can also use your plan in Poland due to free roaming. However, be careful as the law says that “beyond a reasonably high volume of roaming data” companies can charge you (which of course they do). So read the fine print of your plans.