I see nothing wrong with your way of doing it.
It does depend on the type of job you do though. For me, I am on long conference calls so often, that I usually tell my customers email is faster, because I can respond to you right away while I am on my current call, instead of you waiting a hour for me to finish the call. At the very least I can respond with and email back saying "I'm on a call, it should be 30min more. I'll call you back when I am off". I also sit locked to my desk in front of my email all day, so there is no chance of me not seeing an email.
People demanding stuff ASAP and then not leaving enough info for me to be able to quote anything also annoys the crap out of me. Especially when they know better.
To Alcohol !
The cause of, and solution to, all of lifes problems