Some folks have been looking for apps to control who can ring their phones, from restricting incoming calls to only those on their contact list, to only blocking certain callers. I am keenly interested in blocking telemarketers. I can't block everyone not on my contact list, because we often get calls from doctors, service stations, long lost relatives, whose numbers would be impossible add to the contact list.
The Droid 2 has a wonderful feature that will enable you to effectively block a telemarketer from calling you a second time. When editing a contact, way down at the bottom under "additional information", you have the option to send all calls immediately to voicemail. All you have to do is create a contact called "Telemarketer" or "Scumbag" or whatever, and add the telemarketer's phone number. Then check the option to send the caller to voicemail. (The iPhone doesn't have this option AFAIK; but they can select the "no ring" option for that caller.) Get a call from another telemarketer? Just add that number under the telemarketer contact. Run out of space for numbers? Just add Telemarketer2 and select the voicemail option. This is a lot better that the five numbers Verizon allows you to block for 90 days. It is permanent. 95% of the telemarketers will hang up when they are sent to your voicemail.
You can verify the caller is a telemarketer at whocalled.us. I've used this website for years; good information, no strings.
BTW, I highly recommend the app Silent Time (Lite, free), for blocking all calls at certain times while allowing important callers to ring through.