I used to (haven't tested in 2 years) be able to spoof AT&T cell phone numbers, ...

nwilkens on Sept 3, 2014 | parent | context | favorite | on: Detecting Caller ID Spoofing Attacks (2013) [pdf]

I used to (haven't tested in 2 years) be able to spoof AT&T cell phone numbers, and call the same number you are spoofing and listen to voicemails if they didn't have a PIN assigned. Seems like they only authenticated based on CID.