Could be an auto-dialer (robo-caller) that automatically disconnects after a certain # of rings if not answered. Sounds like they are blocking their caller ID and there is very little that customer service can do about that. Best advice is to not answer the call if you don't recognize the phone # or if someone blocks their caller ID. Won't cost you anything or use your airtime if you don't answer the phone (in the US and not roaming internationally).
If I'm annoyed and you're annoyed, does that make us a paranoid ??
Sarcasm is a fine art...
"Don't believe everything you think"
It's not a matter of if you win or lose, it's how you assign the blame