Emails: permanent written record I can refer to later
Can reply in my own time
Low labour
Low resource use
Phone call: Times/dates mentioned will be forgotten often
Active demand of time
I don't pick up because that phone number looks weird but also my phone's vibrate function is weak
High labour
High data cost per information
My shrink's office seems to want to keep billing information and past/present appointments secret. (This also seems to be worse in local industry, everything has to be a meeting instead of a two line email)
Maybe it's some personal preference if it's not too large of an organization? Some people have different communication styles.
Sometimes it's quicker to verbally transmit a lot of information that isn't worth memorializing in the kind of detail or with enough context for it to make sense.
Some places it's a cultural practice around not documenting things so that it's less traceable later.