In the 90's boaters and kayakers could contact the Coast Guard by dialing *CG. But this was discontinued a few years ago. There were difficulties routing the distress message to the proper Coast Guard Station when, and if, the connection went through. Most cell phones are not waterproof and the CG reported that many calls were dropped before the individuals could be located. Moreover, the devices are small, not designed to be tethered to your clothing, and their keyboards are difficult to use in cold water conditions. This assumes you can find and hold onto the device in an overturned kayak.
The Coast Guard continues to highly recommend you carry a VHF radio when you are on water. You should make your initial calls using Channel 16, the international hailing and distress frequency. If you have DSC, activate it. Then repeat the distress call three times clearly: "Mayday, Mayday, Mayday" Follow-up with your name (power-boaters use their registered ship name. It's unlikely that you have registered your kayak, but in that event, I guess you'd use it here too.) An example call would be, "Mayday, Mayday, Mayday. This is kayaker Joe Pike, Joe Pike, Joe Pike. My kayak has overturned and I am unable to re-enter. My position is yadda, yadda yadda... Please send help." If you know your position, state it. Any additional important information can be given once CG contact is established. The CG may ask you to change to another channel.
Why use a VHF radio and not a cell phone? There are too many reasons to list here, but a few quickly come to mind. 1. Sparsity and inconsistent cell coverage (try getting a cell signal on some of the coastal bays some time). 2. Difficulty triangulating cell phones. 3. Water damage. 4. Battery shorting. 5. Loss. etc...
In my mind, the two biggest reasons to carry a VHF transceiver are that the signal is broadcast to many listeners and you are multiplying your chances of being heard, and, if you have a radio with both DSC and GPS, your location is immediately sent to the rescuers.
The latest advance in rescue operations is the Rescue-21 system. It uses DSC and was recently approved for use by the Baltimore sector.
OK, having given you all the reasons why you should carry and use a VHF radio first, I'll also tell you that I always carry my cell phone as a backup. It's carried in it's own waterproof case in a vest pocket on my PDF. It's clipped on.
A few closing thoughts. I read somewhere that you should not call the Coast Guard directly. Instead, you should call 911. I have mixed feelings about doing this. 911 calls made over a cell phone can be routed to places very distant from your location; especially when your location is rural to begin with. I can't imagine a CG station telling you to hang up and dial 911. ... then again, you never know.
Rescued kayaker tells story of survival
Chantilly Kayaker Rescued From Chesapeake Bay