The country I live in, Norway, has a lot of laws regarding advertisement and telemarketing. Unfortunately, as laws tend to be, they're not entirely idiot proof. There's the odd loop hole to allow someone to contact you by phone even though you have registered to not be available for such calls, and then of course there are more than a few firms who simply ignore the law completely.
For the last few years, there hasn't been much I could do about it when some random company has started calling me several times a week. I had an iPhone, and there isn't much you can do on iOS to help rid you of annoying telemarketers. Now however, I have an Android phone, and after just a few minutes of looking around for various apps, telemarketers will never be a problem again.
The exact apps I use are for Norway only and won't help others any, but basically I use one app for automatically looking up known but legit (belonging to people) phone numbers as the calls come in, and another one for looking up numbers that are blocked in the national databases but have been put in a database of telemarketers by a third party. When a call comes in, these apps will check online, and tell me who's bugging me. That way I know if I should pick up or not.
The anti-telemarketer app also has auto-reject built in, but so far I haven't activated that as I'd rather make the decision to block someone myself than have an app block thousands of numbers. What I have done however is add the latest idiotic company that called me to my Galaxy S II's built in blacklist, auto-rejecting every call coming from them. I see some rejected calls in my log as we speak, as the company apparently tried to call me twice yesterday. I didn't even know they had until I checked the logs.
You might ask why I don't just pick up the phone and politely ask them never to call me again. They're people after all, right? This particular company has a lot of complaints online from people who have been sent products despite having told the representatives "no". Between the hassle of sending something back and the fact that most people don't record incoming calls, fighting them can be annoying and hard – unless of course your phone logs prove that you never picked up the phone to begin with.
I also occasionally get calls from companies (normally magazines) that I used to be a customer (subscriber) with, where they're trying to get me back. This is technically a legal way of getting around me being on the "no thank you"-list that all telemarketers are bound by law to check, but I told them explicitly "take me off your lists" years ago without any result. That makes it illegal for them to call.
There probably is the odd telemarketer out there that has something I want, or perhaps just some survey being conducted that I could easily have contributed to without problem. For the most part though, telemarketers seem to be evil people who have no respect for privacy, the law, or anything else – and that ends up ruining it for everyone. The way my phone is set up right now, the only thing it doesn't do is insult them before hanging up. If someone knows an app to do that, let me know. In the mean time, I wish that company that has been trying to contact me repeatedly to sell me lottery tickets the best of luck trying to get in contact with someone whose phone automatically rejects all calls from them, and checks all new incoming numbers against multiple databases. Perhaps you should try someone with an iPhone instead?