Yes, I can understand your frustration. However, I don't understand how calls can get through if you have DND set to always. Do you have the silence set for Always, or only while iPhone is Locked? If always, and you do not have repeated calls on, I'm not sure how some are getting through? Does this happen when you are on another call? And yes, with DND on, you will not receive any other notifications. If you are in the US, then I suggest you make contact to put your number on the Do Not Call list, and you can make a complaint with the FCC. However, if these calls are coming from someone you have done business with in the past, or your number was sold by a company you mistakenly allowed to sell your number, then you cannot even block those.
The information that ember1205 provided about using the call history to block the caller is not correct. If there is no identifying Caller ID provided with the call, you cannot block it.As far as why Apple does not provide the same service Android does, you can provide feedback to Apple to let them know how you feel about that here. http://www.apple.com/feedback and click on the appropriate subject area.