(Assuming Android)
IIRC a sim is a full microcontroller. I'm not sure about the protocols and actual vulnerabilities, but I can say no phone has a trusted or completely documented kernel space or modem. The entire operating system the user sees is like an application that runs in a somewhat separate space. The kernels are all orphans with the manufacturer's proprietary binary modules added as binaries to the kernel at the last possible minute. This is the depreciation mechanism that forces you to buy new devices despite most of the software being open source. No one can update the kernel dependencies unless they have the source code to rebuild the kernel modules needed for the hardware.
In your instance this information is relevant because the sim card is present in the hardware space outside of your user space. I'm not sure what the SELinux security context is, which is very important in Android. I imagine there are many hacks advanced hackers could do in theory, and Israel is on the bleeding edge of such capabilities. I don't think it is likely such a thing would be targeting the individual though. As far as I am aware there is no real way to know what connections a cellular modem is making in an absolute sense because the hardware is undocumented, the same is true of the processor. I'm probably not much help, but that is just what I know about the hardware environment in the periphery.