There's no dialogue here so I can only guess...
The bakery girl was having sex with the guy and accidentally called this girl while they were at it.

I guess she accidentally pressed the button?

Or it seems they were being...intense & she still tried to answer the phone?

My interpretation was something like:

1: X-chan calls Y-kun to make sure he's not late for school again. (Notice how it's the phone at the other end that's ringing?)

2: The ringing phone disturbs Y-kun's sleep, and he answers it in half-awake anger.

3: Y-kun realizes that it's X-chan calling, apologizes profusely, then OUT OF NOWHERE CONFESSES HIS LOVE!

4: X-chan freaks out and immediately hangs up.

Flopsy said:

My interpretation was something like:

1: X-chan calls Y-kun to make sure he's not late for school again. (Notice how it's the phone at the other end that's ringing?)

2: The ringing phone disturbs Y-kun's sleep, and he answers it in half-awake anger.

3: Y-kun realizes that it's X-chan calling, apologizes profusely, then OUT OF NOWHERE CONFESSES HIS LOVE!

4: X-chan freaks out and immediately hangs up.

Actually, the short haired girl is the the main heroine's little sister so she called her and she answered while having sex with the guy from the train who is her sister's teacher.

Wilfriback said:

Actually, the short haired girl is the the main heroine's little sister so she called her and she answered while having sex with the guy from the train who is her sister's teacher.

I thought he was just a salaryman?

HaxtonFale said:

I thought he was just a salaryman?

He is a salary man. It's just that the original fan consensus in the West is that he was the teacher because there was no dialogue. The way you tell the difference between a salary man and a teacher in drawings is that salary men tend to be drawn with a lighter-colored suit, whereas teachers are drawn with darker, grainier suits and/or sweater vests.

Why'd this get taken down?