I have to second this -- the number of spam/robocalls to my AT&T cell phone number has increased dramatically in the last year, and the numbers are never the same. The rise of IP telephony services too easily allow spammers to change their caller ID numbers, so blocking them on the phone or in my account control panel isn't practical.
I'm using NoMoRobo's free service on my landline (at home and our business number) and it's great -- spammy phone numbers that have been crowdsourced-reported as such are automatically hung up on after the first ring. NoMoRobo won the FTC competition asking companies to come up with a way to combat these unsolicited calls.
AT&T needs to give us simultaneous ring on our wireless numbers in order to remain competitive with other providers that are offering this feature.