WHO IS THAT ?

Who is that knocking on my front door ?
Unless I open it I won't know that's for sure
Who is that ringing me on my phone ? 
Unless I answer it the identity is unknown 

Who is that I saw in town just the other day ?
They knew me but them I really couldn't say
Who is that trying to entertain me on the TV ?
I really have no idea who that person can be

Who is that singing that awful Christmas song ?
I think I might know who but I could be wrong
Who is that looking upwards at the dark sky ?
The answer is I