Love this site - I have an iPhone, and if you jailbreak it (Google if you don't know what that means - you need to do it!), there's a program in Cydia called iBlacklist, which lets you BLOCK callers (you can choose what happens when a certain number calls, such as: Voicemail, Busy Signal, Accept and Hangup, No Action [which does nothing], and Accept), and it also has a White List (which is very useful for parents, since you can setup the phone to only allow calls to and from pre-defined numbers.)
I create blacklists in iBlacklist, and put these numbers in - no unwanted calls EVER ring on my phone (OK, well, sometimes one will pop up that I don't have, which is why I end up here, find out who it is, and add them to the proper blacklist.) Bada bing, they always get a busy signal when they call me. :D
Of course, from now one I'm giving them my Google Voice number, which is WAY better; Google Voice forwards to my "real" number, so it's transparent to the caller, but the BEST feature of Google Voice is that when someone calls, I can go into the settings and BLOCK that person/number - if they call again, they'll get a recording saying "This number is no longer in service." For everyone else, it works fine. Plus, when I answer a Google Voice call, I just press 4, and it RECORDS THE CALL FOR ME.
Google Voice works with *any* phone, not just an iPhone. Check it out - you won't be disappointed. Plus you can pick your phone number (and have it spell what you want, if it's available! Mine spells something dirty. LOL)
VERY ANNOYING TO GET CALLS 8 AM ON A SATURDAY WHEN I WORK LONG HOURS DURING THE WEEK TO TRY AND PAY MY BILLS
People need to LEARN TO HOW TO LEAVE A MESSAGE after the recorded message saying " We are NOT available at this time,etc..." AND STOP CALLING every HOUR