Well, by far the preferred way is via e-mail:
If for some reason this address fails (which is unlikely), you can use my backup e-mail address:
Then there is skype:
Unless I am travelling, my computer is usually online 24/7, along with skype on it. That of course doesn't mean that I am always near the computer to hear skype ringing. If I am not answering a call, hang up and wait for me to return the call. Or use the skype's chat service to send me a note.
Then there is the cell-phone:
Depending on where I am (in a seminar room, in bed, in roaming), I might reject or ignore the call. I never answer a "withheld/unknown number" call. If you send me an SMS, I'll make sure to read it at the earliest possible convenient time (which is not neccessarily as quick as you might expect).
Finally, there is the physical address:
Note, though, that this is my permanent residence address in Serbia. It is quite possible that I am temporary living elsewhere at the time when you read this.