Interesting thing about that: if you're curious, dig into the SoCs used for this stuff, and you'll see that's really not about "giving access". The voice handling in most smartphones never leaves the SoC, and the OS just communicates with it like you would with a modem: "call XYZ", "hang up", etc, usually with a familiar AT command set.
Useful side-effect: if the OS crashes, your phone call will typically remain alive. (You probably won't be able to hang up without powering off, though, and definitely won't be able to dial another number.) Or, if you have an original iPhone 3G like mine that's sluggish at the best of times, it doesn't affect the phone call. :)