They just called me for the first time now, the caller ID said Florida. I picked up after one ring and I heard nothing. Afterwards I immediately called back but I got an answering machine with a young woman's voice right away that said 'thank you for calling, please leave a message'.

...and they don't say anything when you answer.

This number calls daily (some times several times a day) and the caller ID only shows "Saint Petersburg" and they never leave a message.