Which GSM system number is hard-coded into the phone and identifies your physical phone hardware to the cellular tower?

The International Mobile Equipment Identity (IMEI) is a one-of-a-kind number that is assigned to each physical phone. The device contains a hard-coded unique identifier. The Product Release Instructions (PRI) explain how to connect your device to the network. A MAC (Media Access Control) address uniquely identifies a network interface card, while an IMSI (International Mobile Subscriber Identity) number uniquely identifies the account holder.