Might be good to get and use an answering machine, with one caveat; have your message to the caller be brief... all the while encouraging them to keep it brief, too:
Can't come to the phone right now... leave your name and number and I'll get back to you. Please don't leave a long message telling me what is wrong with your car. Or email me at fred77@gmail.com
*beeep*
Very cool that you went to the small shop thing after finding out you didn't like the bigger operation.
I think the answering machine could work for you. You could even mention a certain time slot in your message when you return calls... say from 1-2... or after 4:30.