(414) 276 in Milwaukee, WI

Calling leaving call b'day messages on my Phone,these are Robo calls that needs to stop.

They called at 10:46 am on 5/13/15, and again at 6:12pm on 5/14/15. Both times my phone rang once and so I knew it was a robocall. (I belong to NOMOROBO) But both times, within 30 seconds, they called back and the caller ID stated NOMOROBO with the # 1-608-371-6666! (I did not answer the call). The message they are leaving is "This is Kohn Law Firm calling with an important message for you. Call us back at (414) 276-0435, Rick at extension 3083." Why is NOMOROBO number appearing on my caller ID when they call the 2nd time within 30 seconds, and how do I block them???