If you've ever been harassed by a bill collector, or an ex-girlfriend/boyfriend repeatedly calling your cell phone, then you probably already know that your cell phone provider will not block calls from a specific phone number or numbers. Luckily there is an easy way to effectively block calls on your own using a silent ringtone (download here).
Simply download to your phone the silent ringtone above, or make one using a free audio editor such as Audacity. Then...
If you want to 'block' a specific number
simply add the number as a contact in your address book then assign the silent ringtone to the associated contact. You will no longer here anything when that contact calls your phone. Note that on some phones you can add an 'unkown caller' or 'blocked id' contact to your address book by copying it over from your call log.
If you want to block all unknown numbers
simply set the silent ringtone to be the phone's default ringtone, then assign a real ringtone to each contact in your address book.
Keep in mind that you will still see the calls in your call log (obviously). The second option is great for avoiding bill collectors. I'm keeping comments open, so let me know what you think.