Is it necessary for a grad student to respond to phone calls, as long as they're responsive on email? I am asking this for a friend who hopes to enter grad school for a PhD next year in science.  This person has a cell phone but almost always turns it off and has no voice mail (meaning there is no way to leave a phone message). I think not answering phone calls or being available by phone would make it difficult for my friend to be successful as a grad student, esp. as a RA or TA.

My friend has a variety of reasons for not wanting to have phone on or voice mail set up that I will not specify here. 

So I am looking for thoughts from grad students or professors.
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.