Is there a list of sensibly built phones available? I'd like to buy a phone where the modem and SIM do not have access to main memory (AIUI most phones use a single-chip SoC with a built-in modem).
Main memory is rarely encrypted, unless you have special security features in your CPU to do so. Only the disk is encrypted; main memory is vulnerable while running. Also see https://en.wikipedia.org/wiki/Cold_boot_attack
So you don't want any hardware to have access to main memory if it doesn't need to. For instance, you can use an IOMMU to ensure that devices can only access the specific areas the OS wants to allow them to DMA to/from, not all of memory.
