Depends how you wired up the batteries and what is actually supplying voltage to the phone. If it uses a regulator, zener diode, or a small DC to DC converter, then the voltage output will be constant.
If you are hooked up directly to the batteries and they are all in series, that would still only be ~4.8v and most cell phones take 5V input. (Though I have heard of very cheap car adapters that run the 12V straight to the phone, and in that particular story the iPhone was able to handle it with its' internal charging circuit.