Caleb's eyebrows go up a little at that; it is an interesting detail just because it's so different to his general collection of experience. Even in his neighborhood just about everybody had a phone, even if it was a cheap, shitty pre-paid burner-- in fact a person was much more likely to have a cell phone than he was have, for instance, have dental insurance! Good times in America.
Anyway. He takes the phone from her long enough to note the number - which is mental, and rapid; he's good with remembering those - then palms his own from his pocket and thumbs out a message short enough that the text received signal chimes out before he hands it back to her. "There, now you've got mine. One-two punch, see?"