Is it necessary for a grad student to respond to phone calls, as long as they're responsive on email?
Anybody who wants to participate in scientific life, even as a grad student, needs to have some way that they can be reliably reached in a timely fashion. They also need to provide that information to those who need to reach them (e.g., their professor, students if they are a TA).

Whether their recommended best means is by cell phone, by email, or by instant message, however, matters much less.  In fact, the current generation appears to be moving away from voice calls, so cell phone availability is less likely to matter in any case.

Thus, as long as it's easy for people who need to reach your friend to do so, the particular choice of not having a cell phone on should not be a problem.