According to what I found via Google, area code 493 is in France (Cannes or Nice). 800 is the old "toll-free" area code. Personally, I don't answer calls from people I don't know, and I certainly don't call them back. (Given how easy it is to spoof caller ID info, I wouldn't trust the numbers shown, anyway.)
If they're really not random, call your cellular service provider and tell them you're getting harassing calls. At the very least, they should be able to block them.