I think we need to look at both sides of the scenario.
I understand the situation but the garage may be reluctant to confirm the diagnosis based on a phone conversation.
I ain't trying to defend the garage in this case (far from it) but I constantly get people phoning me up for advice regarding their car problems and whilst I may be able to give advice I would not be in a position to know exactly what the problem may be without seeing it. I don't charge for diagnosis but I'm beginning to think I should.
Better still I have loads of filters, plugs, etc lying in my garage because people failed to turn up for work to be done and I know why - they tootled off to another garage armed with my information.