You have to free the phone.
This means that the phones have a restriction so you only use the SIM cards from the same company.
Even though this shouldn't be allowed, because they can't MAKE you use only their company services.
But anyway, to free your phone you have to find the code for the type of cell phone that you have.
It requires a little search depending on the brand, Nokias are the easiest to find the code and to free.