On the iPhone, look at your recent calls. Click the blue circle with the i in it on the right of the number / call you would like to block; this opens the Info screen about that call. Scroll to the bottom, click Block this Caller. (this is how it works on the iPhone 6; guess they are all the same/similar).
The same thing happen to me. A voice mail left regarding lawsuit on me, asking me to call back 202-864-1295. This is not the first call i got claiming themselves as irs, so it is my guess that this is a scam. After reading the posts from others, I can be most certain. First, i do not think that I owe any taxes. It is paid up every year. Secondly, i work for the government, and I know for important things like that they dont just call you. They will send you a letter. Third, the caller is a machine, not human.
A very reliable online site like USpopularnames.com will let you experience visiting a dependable web-based community. In case you need verify a call using a reverse lookup directory, this website lets you complete the process. Likewise, you can post your comment and let other people read it.