We use walkie talkies. Even with fulltiming, the batteries last at least a month or two unless you leave it on. We've had different ones over the years - they all worked pretty much the same. Really, you are only 40 feet apart and only use them for a max of five minutes at a time. Pick one. You can get a pair with rechargeable batteries but you have to remember to recharge them - we never do.
Cell phones don't work if there is no cell signal, the radios do. Too often, we find ourselves too far from cell signal to use them. Also with the radios, if I wander away, I can carry one for 'just in case.' :)
