Small point but this was an addition to Royal - the followup phone calls weren't in the base game - and are largely present to give you more chances to gain confidant points within the advancement events and not have to commit too many non-advancing time slots.
.. This does make one of the game's other things pretty pointless, which is the large list of items that only exist to be gifted to confidants; if you're using your time well and making optimal dialogue choices in the advancement scenes you'll almost never spend time with somebody that isn't ranking up, and so never have an opportunity (or really a reason) to give them said gifts.