Best rule to follow, only answer if it's in your area code. Most spammers don't bother to match area codes, but your co-workers do.
Edit: Oh neat! Apparently it's illegal in the US, but legal in Canada (where I am currently located). So it's not the robocall not bothering, it's it being flat-out illegal. https://en.wikipedia.org/wiki/Caller_ID_spoofing
I still answer most calls from my area code, but if I suspect it's a robocall I just pick up without saying hello. If it's a robocall, there's generally a lengthy pause and then you can here someone connect (at which point I hang up).