SIM cards don't know anything about the network they are running on, this all happens inside your terminal. They hold an IMSI (=contract #), a set of private ke

SIM cards don't know anything about the network they are running on, this all happens inside your terminal. They hold an IMSI (=contract #), a set of private keys, and some utility applets. SIM cards don't even know their own phone number. As for running a known-secure firmware: in the end a SIM card only authenticate you against a Mobile Operator with a pre-shared key. If you believe your SIM is running a non-secure firmware, how can you trust your Mobile Network Operator not to do fancy stuff on their network behind your back?