How to block a number on iphone 5?

Okay i have AT&T and i got a number blocked through them paying the extra 5 bucks a month and everything, turned off i Messaging on the phone itself and i've been good for a while no texts or calls but then i got a picture message from the number. Checked online to see if the block was still active and it is, same with i Messaging. Does anyone know how or why this is possible?

Jailbreak and install iblacklist. Its blocks everything

Shawn, blacklist them. I used www.seeblockednumber.com and their service worked great. There are lots of options out there.