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 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). My phone is also connected to various online messaging apps like Viber, WhatsApp, Telegram, Skype, WeChat, etc. You may try to contact me using any of these, with variable success.
Finally, there is the physical address:
Note, though, that this is my permanent residence address in Serbia. It is quite possible that I am living elsewhere or am not at home at the time when you read this.