theory of GSM says that in any registration ( mobile to the network ) IMEI (only once,after power on the phone and entering pin ofcourse) and IMSI (always: after PIN,SMS,CALL,DATA etc ) given to the network.
the point is if operator HAVE THE BLACK LIST OPTION ENABLE.
for examble here in greece this option is disabled.only VODAFONE use this option for 1 year old PROTOTYPE phones ( given by vodafone as sample)