imagine:
kashitarou and amatsuki get lonely because they miss each other, seeing as they don't hang out as often anymore. they can't really see each other since they're really busy lately too.
and so what they do when they have the free time is that they call each other. then, at the most, every call they have lasts so long (but not that long) they forget where the time went, because they're having fun talking about what they're doing and how their day was and laugh and giggle over the dumb things that happened over the months that passed.
("kids crowded me when i was walking around with my guitar earlier at the park, haha! they wanted me to play some songs~" "that's cute! i wish i could've seen you, something like that doesn't happen everyday!")
and as for why their calls last for so long, it's because they don't want to hang up. they'll only end up missing the other more.