HR folks, I know your job is tough. I know it's way more than just hiring and firing. It also involves employee benefits, payroll, worker complaints, and endless number of laws and mountains of paperwork. You definitely have a lot on your plate.
However, maybe you could spare a few minutes for a bit of a marketing lesson. Every time you don't return a phone it hurts your brand. Someone feels mistreated and they will not buy your goods, despite how excited they sound in their cover letter or interview.
In addition, you expect the candidate to take the time to write personalized letters and resumes, but you send back a very generic "thank you for your interest...we're going in another direction" letter or email.
While I have a shoebox full of generic letters and a head full names I vow not to forget, I say this as a marketer. Candidates take enough of an interest in your brand to say "Yeah, I'd love to work there" so chances are there is the potential for customer behind that resume.
For your brand's sake it's time to put the human back in human resources. A bridge burns both ways and it won't help your brand by being the bully.