How do I call a Poland land line from my USA cell phone (I have Verizon international plan added for trip) while in Poland?
Have you checked the linked website? As I understand it your phone remains US based so I would use US to Poland as the choices.
Per Verizon's instructions, for calls within Poland from your Verizon phone, dial +48 and the local number you are calling. You do not need to dial "11" as the "+" is the instruction to dial international.
Since we are talking about cell phone, may I know which providers in Poland have stable and more extensive WIFI coverage? Orange? I will stay in Poland for 21 days, wonder is that a prepaid SIM card package for such a period of time? Thank you
Exactly the same as you do when the phone is in the USA:
+ 48 {area code} {number}
Your phone remains a US phone with a US number. "48" is the country code for Poland.
If you store numbers on your phone in the "+ {country code} etc." format they work no matter which country you are in.