Computers have revolutionised the making of sales calls but the short cuts they take now make it absurdly easy to not talk to them. Sales call companies use computers to make thousands of phone calls. If a live person actually answers the phone the call is then routed to a sales person and you can hear this as the line is initially silent and then you can hear the click of someone coming on the line. If you're fast you can detect this delay and hang up if you don't get a response on the first hello.
But that's more work than I want to do. I got caller id because I really want to know who is calling and my cell phone has it and I love it. I don't like paying for it but it's proved invaluable to not have to have unwanted conversations with companies/organizations that I have/had previous relationships with but don't want to have a conversation about it right now.
Sales call companies block their number in a host of ways (google on "unavailable caller id") but they almost always do and I've never seen a legit call come up that way. Even if it was a legit caller they'd leave a message. We now have a joke in our household about how "unavailable" always calls and never leaves a message. Sometimes the number shows up but the name does not which provides the most information (example below.)
Of course, the first step in avoiding sales calls is to add your number to the do not call list. I've heard people claim that sales companies ignore the list, but that hasn't been my experience, though it's not like I'm answering their phone calls either.
If you do get a phone number with the name as unavailable appearing, there's this handy site called 800 notes where you can enter a toll free number and find out who owns it and any comments people have about it. For example I just got a call from 888-858-9823. I looked it up on that site and found out that it was Chase - a company I haven't dealt with in years but oh well. One poster said that if you called the number there was an option to remove your number from their sales call list, and they were right. It took a little while but it finally let me enter my number and promised to remove it. I'll have to try this on the other numbers too though most of the calls are from my schools (UCSB and Cal Poly) or the people they hire like Telefund. Though now that I look at Telefund's website I see the it was more likely The Sierra Club or the Democratic National Committee. The trouble with all of these groups is that they are so hard to say no (high guilt factor and they know it) to even though I never give money over the phone.