Personally, if you think they might be calling regarding your account, just call them back using the customer service number on your bill/at the official website (or go to a local site and talk to a rep there). They should have a note in your account records that they've been trying to reach you and why if it's important.
With all the spoofing that goes on, though, I'd be more inclined to think it's a scammer trying to trick you into picking up the phone, especially since they don't leave a message if they don't get a live person.