The unknown number that called might have been a complete coincidence at that time. Who knows.
http://www.mangago.zone/read-manga/color_recipe/mi/c25c6388b7db03def513f5c6776ce126_chapter-3/3/
His hand is in his pocket here so he might have dialed his phone if it was in there.
http://www.mangago.zone/read-manga/color_recipe/mi/c25c6388b7db03def513f5c6776ce126_chapter-3/3/
The second panel on the left shows a slight rattling effect of fuku's left shoulder, indicating movement. He could have dialed from his pocket here as well.
The bird's eye view here and the natural posture of the page in the previous link suggest the panelling was seen through a more realistic eye, where his movements are so smooth, there is no reason to be attentive to what WE know to be of interest. Either on or off panel he is dialing the phone somehow, but just like the plot of the series as a whole, it's no so easy for people to peg fuku as someone with a mental illness; harada-sensei decided that his manipulation methods even escape our view (the reader). Indeed, it is the assistant who tells us the twist to the story, we are constantly at an angle of confusion ourselves, as though we're seeing things exactly as they would have appeared to a person who existed in the universe. Essentially, we gain no privileged perspective as the reader. And i think we're intentionally not shown how he calls in these two instances because he's so manipulation savvy, a normal person wouldn't notice him calling.

Remember there was a time when Fuku was with Shou-kun and that unknown number phoned and Fuku picked up...who was that then?
One of the most intimate sex scenes I've ever read!
The last couple of pages with Kihara confused the crap out of me...