Polish mobile numbers have always 9 digits. It is enough if you dial just these 9 numbers, without any area code.
When it comes to stationary telephones, their numbers have 7 digits, but you must dial it proceeded by an area code for each city.
For Lodz it is 42. Therefore the number for taxi is (let's recall it): 42 6 400 400 (it is not essential to dial +48 in the beginning, it is code of Poland).