Are ESN and SIM related?
I am venturing into repairing and reselling iPhones as refurbished and need to get a better understanding of the ESN and SIM. Is the SIM card the GSM version of the ESN (CDMA) number? Can a GSM phone really be locked by the carrier if the SIM card can just simply be replaced with a new one? And is the ESN really the MEID number? When I activated my wife's and my iPhones, the carrier asked for the MEID, not ESN.
The ESN number is the GSM equivalent of the IMEI number. In regards to GSM carriers locking phones, yes they can lock just the phone. This is mostly always the case with most major GSM carriers. You can only use a carrier-specific SIM card unless the phone is unlocked. Once the phone is unlocked, you can use any GSM SIM card (provided the phone is compatible with the network bands.)
In regards to ESN and MEID numbers, here's an excerpt from wikipedia:
As ESNs have essentially run out, a new serial number format, MEID, was created by 3GPP2 and was first implemented by Verizon in 2006. MEIDs are 56 bits long, the same length as the IMEI and, in fact, MEID was created to be a superset of IMEI. The main difference between MEID and IMEI is that the MEID allows hexadecimal digits while IMEI allows only decimal digits – "IMEI shall consist of decimal digits (0 through 9) only"