Do all operators offer open phones?

Many GSM phone service operators already allow any mobile phone to be used on their networks with a valid SIM and service agreement.  Some even sell SIM cards separately and directly to consumers for their use in any phone.  However, there are many levels of openness and each operator has its own policy.  Ask the operators for SIM-only packages if you want a cell phone and you do not have service with a SIM yet.