If you have a smart phone with the android market download the call block app, anytime someone calls and you dont want to answer add the number to the app and you can set it up to when certain numbers call it goes straight to voice mail, very handy!
Good idea. I do something similar.
Because I would get so many numbers from various area codes, with similar but not the same numbers, I have turned off the ringer globally. Then, I manually turned back on the ringer for absolutely everyone in my contact list.
Now, whenever my phone rings, it's because I know you. All other calls are deselected and do not ring. There is rarely a message. Sometimes, there is and I simply delete it.
If a new, unknown but legitimate call comes in, the caller will inevitably leave a message. I may then choose to add the number to my contact list and turn on the ringer for that number.