I'll pose the following scenario in order to ask this question.
During the recent war between Israel and Hamas in Gaza, a man was called up to duty. On one of the days that he was found in the South, his son was suppose to have his Bris. He was unable to make it to the actual Bris. He asked if he may listen to the Bris on the phone and when the time it happens make the bracha. I heard this shailo was actually asked by the Beis Horah of R' Wosner and the Rabbonim there said he may not make the bracha. However I heard that other poskim argued on this and said that since he knows that at that time the bris was happening, he could therefore make the bracha.
My question is as follows. In any case where the Father wasn't able to make the Bracha at the time of his son's bris (and he was the one who appointed the mohel to be the shliach to make the Bris), how long after the bris happens may he make the bracha? Must it be made right when the mitzvah is done? What about if he's not there; can he make the bracha when he hears about it happening (through the phone)? What about if he knew the Bris was supposed to be on a certain day and he didn't find out that it actually happened until a few days later; may the bracha be made then?