Would you use ser or estar to talk about physical location?

estar.

(I once said, on a doorphone in Spain, when asked 'Quien es?' (Who is it?): 'Estoy Fred' (I'm Fred) - meaning 'I'm Fred, here at the door'. Puzzled silence. 'Soy Fred', I ventured. ''Ah, Fred, estas alli!' (Ah, Fred, you're there!); and the door opened).