I use
Root Call Blocker Pro. It hangs up on the caller before the phone even rings.
http://goo.gl/nrve5
Only problem is it doesn't have the spammer database that
Mr. Number has, so I use both.
http://goo.gl/QyITd
Mr. Number warns me if a caller is probably spam -- from an online database wiki -- and that first time it goes to my voicemail (except they never seem to want to leave a message). Then, I add that number to my Root Call Blocker Pro app, and that number never calls me again!
I would use Mr. Number exclusively but with the version of Gingerbread we have, Mr. Number can't "pick up and hang up" like it could on my LG Optimus. It can only send to voicemail on the LG Esteem.
Root Call Blocker Pro blocks calls from even completing. It disconnects before the phone call even completes and rings. But you have to be rooted for it to work. You can also block "private numbers" (those who don't show their Caller ID).
Root Call Blocker Pro is a paid app, but for me it's worth it. Mr. Number is free and has the spammer database, but it doesn't work on our Esteem the way that I would like. So, I use both.