I have a friend who has an ingenious padlock/steel box thing that she attaches to the underside of her car to leave her keys while shore diving. I need to look those things up, because it's a great idea. Sort of like one of the real estate broker key boxes.
When I dive with her, my keys go in her trunk. I have an electronic key with alarm remote I have to keep dry, if it was a regular key I would just keep it in a pocket (well-secured, of course). Ditto if I'm diving with someone else with a regular key... my electronic stuff goes locked in their trunk, and they take their regular key with them, which isn't really affected by water.
When I dive without a better method (for instance, if we both have electronic keys to keep dry), I do have a small dry bag with triple heavy-duty zip-loc seals that has kept my keys dry down to about 90 fsw, but I prefer not to risk it.
Cell phone and any other electronics stay on shore.
The only relatively certain way to keep such things dry would be a waterproof canister of the type used for the battery pack on a can light. I believe there was a thread somewhere on this board a few months ago by someone wanting to bring a VHF radio down with him for similar reasons, you might try a search. Even those can flood, but you might be able to pick up one big enough for a cell phone on eBay for cheap if you purchase one with a known-defective light head. Another thought, frankly, is a largish, older UK 8-D flashlight. The battery compartment on that might be big enough for a cell phone or GPS depending on the model, and those are supposedly rated to 300' or so. Somewhat bulky, however. There are no easy answers to the mix of electronics and water...