|Think about Loose Coupling|
Has to be Email for me.
I have a cellphone, but only really have it in case the car breaks down (so I don't have to walk miles through rain to call the recovery service). Sometimes I even leave it switched on when I'm on call, though it's switched off once I leave call.
Most of the time, I have it turned off and redirecting to my land phone.
My land phone, I screen, as most of the time, I'm too busy doing other things to take the call there and then. When I'm busy, I'm usually online on my PC.
Being online, I check my mail frequently. :)
Smoke signals would only set the fire alarm off, which would make me irate.
A carrier pigeon would certainly make a nice meal afterwards, tho I hate the plucking part.. Takes too much time...
Telepathy would rock, but then, how do you tell telepathy from the 'voices in your head'??
So, plain old email wins the day. :)