[sklnd]

ESN is equivalent to IMEI in GSM/UMTS networks. IMEI is tied to equipment, and never changes, just like ESN.

CDMA networks use IMSI to identify subscribers (as opposed to the equipment serial number) just like GSM/UMTS. IMSIs are provisioned on to the phone by the carrier with special software supplied by the phone manufacturer, instead of being supplied to the phone by the phone asking the SIM for it.

There's no technical reason why CDMA networks can't use a removable card to store subscriber data, and indeed there are a few standards for doing so (RUIM and CSIM) and a few carriers that use them.