No application on iOS can block "No Caller ID" or "Unknown" calls on your phone. Unfortunately, this is a limitation of iOS and Apple does not offer developers a way to block these types of calls.
However, the good news is that we are also the creators behind TrapCall.com - the world’s first and most popular service to unmask blocked calls, so you can actually reveal the number of these callers. By revealing the number hiding behind these blocked calls, you can get to the bottom of both WHO and WHY this caller is calling you and get them to stop. TrapCall has been around for over 8 years and has over 750,000 satisfied customers, with over 14,000,000 calls unmasked!
* Please note that you can't use RoboKiller and TrapCall at the same time.