He worked for AT&T at the time, right? Those corporate PBX systems had all sorts of crazy features which people mastered by pounding the 12 keys really fast. And he was probably on the bleeding edge of that. (In many places digital voice mail commonly predated email.)
edit to agree: obv Stroustrup in 1990 was not talking about your cell phone.
edit to agree: obv Stroustrup in 1990 was not talking about your cell phone.