What is SIM Card?

The card itself is a UICC, a small secure processor running a USIM application. It holds the IMSI - a mobile country code, a mobile network code and a subscriber number, fifteen digits at most - along with an ICCID printed on the plastic and a secret key that never leaves the chip. Authentication works by challenge and response. The network sends a random value, the card computes an answer using the stored key, and the operator compares it with its own copy. Neither side ever transmits the key.

Form factors shrank from the mini-SIM through micro and nano to the soldered eSIM, an eUICC chip that accepts operator profiles downloaded over the air. Proxy farms pay attention to that last one, because a rack of modems can be moved to a different operator without anyone opening a case. The APN configured on the SIM or the modem decides which gateway hands out the IP address, and a wrong APN leaves the device registered on the network with no working data path behind it.

Whatever plan sits behind the SIM shapes how the proxy behaves. Prepaid bundles stop or throttle hard at their cap, so an endpoint that ran fast all month can crawl on the 28th. Geolocation databases read the operator's registered ranges, which is how the SIM's home network sets the apparent country. A roaming SIM may egress in the visited country or back through its home operator, depending on whether the traffic is home-routed or broken out locally, and that catches people who buy local SIMs for a market.

Several things break in practice. A SIM PIN left enabled stops the modem attaching after a power cut unless the host supplies it on boot. Carriers deactivate SIMs for non-payment, and some suspend them for traffic patterns that look nothing like a handset. In a dense modem shelf, heat and worn contacts produce intermittent detaches that read as network faults until someone reseats the card. Keeping an inventory of which SIM sits in which slot saves hours when one of them starts misbehaving.