They called my cell phone, and all I heard was an automated "Good bye"

Robo-call from Direct Buy, a notorious company that has one of the worst reputations of any company in the USA.  They are not only reportedly abusive, corrupt, and deceitful in their business practices, they also wantonly violate the Do Not Call list.  I contacted their corporate office but they don't care.  

I'm pretty sure there is regulation against automated calls to cell phones without express consent of the customer.

Nope.  You're assuming everyone knows which numbers are cell phones and which ones aren't, which isn't true.  All the same, the Do Not Call registry applies to all phones.  Register your number there!

I have gotten several call when I answer all I hear was good bye it is strange it must be stopped