Kaspersky's Mobile Security does an amazing job of it.. You can set your phone so that only people in your contact list can call or message you, or you can block individual numbers.. It can also lock out your phone if your phone gets lost, and remove any bad software you accidently put on your phone. It has a remote control option so you can send an SMS to your phone with a pin# and it will respond to your commands even if somebody else is using it .. Heck, it can even respond with the GPS coordinates of your phone in case you drop it somewhere and need to go find it...

I pay for it, IMO, it's worth it.. I just renewed it last month.

Is it android phone, just shake it to block call. You know how to do this. Maybe need the "Jerk Off" app?

My Samsung Galaxy has a blocking feature out of the box. It's available from the Call Log lists. I would think it is a standard Android feature. 

This is a good question and I would like to know the answer too. I constantly get spam callers but I always pick up because Im afraid that it's important. I really want to block a couple of numbers but my iphone settings doesn't seem to have this option.

You don't need a special app for this. Generally, you should be able to go to your settings menu and see "call options" or something like that. Then maybe you have "call reject" or "blacklist" or something similar.

Edit: I have an android phone, so not sure what it should be like with an iphone...

you wanna block what calls? numbers of your contacts? (in android, yes you can. in iphone, get an app).

or numbers of unknown numbers, spam, scam calls? (how is that supposed to be possible?)

a bit more information would be necessary to answer ur question precisely...

No why?