“There is one key used for communication between the operators and the SIM card that is very well protected, because that protects their monetary interest,” Nohl said. “The other key is less well protected, because it only protects your private data.”
Source: $15 phone, 3 minutes all that’s needed to eavesdrop on GSM call | Ars Technica
Just because you pay for a service (or device) doesn’t mean you can trust it.