Your problem is that you're treating words such as "function" and "call" as if they had meaning outside of the language itself (or, more specifically, outside of the C abstract machine), when the point of the compiler is precisely to melt away the language parts of the specified program and be left with a concrete program that matches its behavior. If you view a binary in a disassembler, you will not find any "functions" or "calls". Maybe that particular architecture happens to have a "call" instruction to jump to "functions", but these words are merely homophones with what C refers to as "functions" and "calls".

When you "call" a "function" in the source you're not specifying to the compiler that you want a specific opcode in the generated executable, you're merely specifying a particular observable behavior. This is why optimizations such as inlining and TCO are valid. If the compiler can prove that a heap allocation can be turned into a stack allocation, or even removed altogether (e.g. free(malloc(1ULL << 50))), the fact that these are exposed to the programmer as "functions" he can "call" poses no obstacle.