The very same thing happened just now to us. We thought it was someone calling to harass us about how rotten OU was doing - so I called it back several times, but it always rang once and then went in to the beep-beep-beep that means "overload" (too many calls coming in :()
I did not answer the call from this number and they did not leave a message. I blocked my number and called right back and it said Verizon Wireless National Customer Service... I don't have Verizon as my provider, so??? I called back a few minutes later and got the busy tone as someone else has mentioned they got.