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)
Reasons they might prefer phone calls over emails:
Email is a written record you have and can potentially use against them. They can tape phone calls but they know you're unlikely to tape them.
It is easier to upsell something in a phone call than in an email.
If there are more emails than there's time to answer, they get a backlog. If there are more phone calls than there is time to answer you get a backlog or give up altogether.
It is easier to push a customer towards the resolution they want in a phone call than in an email.