Every SIM card has a unique ID; the International Mobile Subscriber Identity (IMSI) is used to identify the user of a cellular network and has a unique ID associated with a specific cellular network. SIM ID is stored as a 64 bit field and is transmitted from the phone to the network. The Home Location Register (HLR) is locally copied in the visitor location register.
The IMSI mobile network interconnects with all other networks. The Global System for Mobile Communication (GSM), Universal Mobile Telecommunications System (UMTS) and Long Term Evolution (LTE) network phone number is provisioned in the SIM card.
When turning on a mobile phone, and when the phone is turned on, the phone registers its SIM with the network. The mobile network performs authentication and authorization against the HLR and marks the IMSI as being active. If the IMSI is marked as active the authorization will fail, the network will disallow the mobile to access its services.
SIM card cloning
Power your phone off and remove the back cover. Next remove the battery from your phone. Remove the SIM card's carrier tray, and pop out the SIM card. Copy the IMSI (International Mobile Subscriber Identifier) number. An IMSI is from eleven to fifteen digits long. The IMSI is imprinted on the SIM card. i.e. IMSI: 32112345678
Now that you have the IMSI number you'll require t he authentication key (ki), unique to your SIM. Use the SIM card reader that you will insert into the SIM card slot. SIM card readers can be purchased for around $10 online. Do a Google or Bing search for "SIM card reader" to locate a seller.
Connect the SIM card reader to your SIM and your PC, the ki number will be retrieved and copy the entire contents. After the duplication is complete the new SIM will be identical to the old. Pop in the new SIM card and power your phone on to use.