If my field, I tend to get a fair amount of out-of-the-blue sales calls. I have caller ID on my office phone, so I can usually tell when a call is a sales call, thus don’t bother to pick it up.
However, if they call me twice (usually spaced a few days apart) and both times I let their call go to voicemail, then they call me from a cell phone number…which I pick up because those tend to be legitimate calls. 
Has anyone else encountered this tactic?
I haven’t come across that tactic, but I do feel inundated by people trying to sell things or collect for charity at the moment and it’s driving me crazy. It seems that someone comes to our door at least twice a week these days.
Force Flow,
The sales call that get to me are typically those passed on by our front desk person. Otherwise I don’t answer. It hasn’t been that bad (maybe 1 every couple of months) but I’m on the verge of telling them to screen these calls before they pass them along.
Shawn
When I realize I’ve got a call like that, I just put the phone down and get on with things, then hang up once I hear the beeps.
Get a rottwieler. Problem solved. 
I simply don’t asnwer to phone numbers that I don’t recognise. It is not only for sale calls, that’s the truth but still, if they don’t have a legitimate number and don’t leave a message, I don’t asnwer.
I also tend to search the unknown numbers on the internet.