Is there any research into which and why people prefer filling contact forms/writing e-mail to calling by phone?
I want to check my hypothesis that wealth people over 30 y.o. prefer phone call over any contact form
This first used contact method table looks like some good research based off the Government Contact Center Satisfaction Index - which I think is referring to this: http://www.howto.gov/training/classes/government-contact-center-customer-satisfaction
First used other contact method besides phone to reach government agency
Indicating likelihood of calling goes down with more income - against your hypothesis.
This blog post looks at the breakdown of contact for age ranges:
Which implies that the major shift (increase from 24% call to 35% call) comes around age 45 not 30.
I don't know about who and why prefer filling contact forms or calling, I guess it depends of the context (as well as the user). Ex. For ordering a pizza you would use the phone (faster) and for a suggestion/feedback maybe the contact form.
I both cases, I would say that mobile user prefer calling than start filling a contact form from the mobile device. In case of the desktop, writing is more friendly.