have been called at least twice this month.  Ditto last month.  No messages left.
Caller ID was just "I".

The call is from a charity, usually breast cancer, and they want your money. If you have ever given money to a charity, you have been placed on a list that is circulated among many charities and they will all call you eventually. Ask to be taken off their list, but don't expect it to happen any time soon, even if you are on the national Do Not Call list.

I get two calls per day from this number but there is no one on the other end when I answer.

This autodialer program has been calling me at least three times a day on my landline.  If I answer it, there is a very low volume recording about breast cancer or something, I hung up after a few seconds of the sales pitch.  Obviously it doesn't matter if you are on a do not call list.  These people still call.

nothing to say calls from just after 8 a.m. till almost 9 p.m.