The main carrier in northern Houghton and most of Keweenaw County (essentially from Twin Lakes to Delaware) is AT&T. You will find just a few dead spots now days, very few of them on the trail. Once you get up around Eagle Harbor, Copper Harbor and the tip of the Keweenaw, you are out of luck with AT&T, but who ever bought Alltel has a few towers in those areas.
I also get OK coverage with my ATT phone when I travel through Marquette, Ontonagon and Gogebic Counties.
As Dave mentioned, you might want to look into the Spot system. I have one that I use while out west. It is satellite based, so no dead areas as long as you have a fairly clear view of the sky. You cannot talk on it, but you can send a pre-designed message that says you are ok and where you are, or sends an SOS with your location or, with a smart phone and their app, you can send text messages. Pretty nifty safety device. Plus you don't need to worry about missed calls!
-John