So the sender, who's listed as "Dad," calls the recipient "Dad" as well? My theory is that recipient adopted a child at a very early age - let's say he adopted a 2-year-old when he was 5 - and to help his son cope with the scant age difference between the two, the father just told the son that they were both each other's daddies because they had each conceived the other at the same moment in some quantum wormhole. The dad went to such great lengths to preserve this white lie that he listed his son as "Dad" in his contacts list. In this exchange, however, the dad finally tells his son the truth.
Judging by the picture, whenever his mom calls he's seized with panic and is shown desperately trying to remember whether he forgot her birthday (again).
When his dad calls, he puts on his Serious Man-Face and gets ready to field Serious Questions about his future and whether he's started saving up for a house yet.