You’d hope Nishikata getting a cell phone would be a chance for him to grow up a little, but he only sees it as the trump card in his futile quest to successfully tease Takagi.
Perhaps he grew up a bit in spite of his intentions, since it doesn’t take long for him to cave and spout his whole plan to Takagi, who saw it coming from miles away. In any case, Takagi wants his contact info anyway, so they exchange it, and she promises to use it.
For calling him out on his attempted tease, Takagi appeals to Nishikata’s general honor by telling him he should make it up to her by helping her with classroom day duty; Nishikata does not resist. He’s tired because he couldn’t sleep after watching a scary video on his phone.
When he sends it to Takagi, she first teases him by showing him a picture of a woman in a bikini, asking if that’s what he sent, then watches the video, only to not be that scared at all because it’s not nighttime (and because let’s face it, it’s a dumb video).
That night, at the same time Nishikata is about to send her another scary video, she calls him to head it off, again correctly predicting his behavior (though that isn’t that hard at this point).
What results is what she wanted all along: to simply talk on the phone with Nishikata, for the first time at night, even saying it makes her heart flutter. Because of that, Nishikata can’t sleep a second straight night, though for a totally different reason!
Nishikata’s efforts to take a phone photo that will beat his friends’ lead him to beg Takagi to do her patented funny face so he can snap it; Takagi’s cost is high: he’ll have to do “whatever she says.” When she finally does the face, Nishikata isn’t ready. Then Takagi pulls out a toy snake and snaps three pictures of his reactions.
While she’s laughing at those pics she’s so happy to have taken, Nishikata tries to take her picture, but she snaps immediately into a normal peace-sign pose. Nishikata proposes they each delete the photos of one another, but Takagi refuses, and tells him not to delete her picture off his phone, though also asks him not to show it to anyone else, as it’s “embarrassing.”
While he may have thought he’d have a new weapon to even the odds in his cell phone, the truth is Takagi is far more adept in its use. It’s a tool, after all; it’s all about the talent of the person wielding it. So he’s merely given her another avenue to “tease” him, though it’s mainly a means for them to get a little closer.