Cellular phone cloning

Usually the mechanism is matching IMEI with the phone number. However GSM networks, which use a SIM card embedded with the phone number, don't have such a matching mechanism. So you can use the SIM card on different phones, thus phone numbers are not fixed on devices.

Here's a Wikipedia article to explain that. But, this subject is not relevant to cryptography.

Authentication itself is, of course, achieved by a simple cryptographic function using an authentication key. This article explains it a bit.

In cellular phone IMEI not used for authentication. authentication key is a key naming Ki. Google is helpful.Refer to these links: 16 in 1 super SIM

For GSM security see GSM Security and this other document on GSM security.