How does the caller know if you're the type to use voicemail? (I never check mine, if it's urgent you would have called back) How often will you check your voicemail, if at all? Hourly? Once a day? Maybe the call is semi urgent, it can wait an hour but not a day.
Key point: knowing why you're not taking the call is valuable information to the caller.
Send an email? Most voicemail systems will have a little flashing light on top when you have a message waiting.
If I'm trying to write a tricky piece of code and you keep calling me, a) I will be annoyed and b) my mobile will go off, my email client will go off and the land line will be unplugged from the wall.
What are you going to do now? Put on a gorilla suit and come jump up and down on my desk? Learn some class.
You are correct that that information is valuable to the caller.
To me that's, in general, not an acceptable information flow. It's a side channel that reveals information that I don't think people in general need to have.
Key point: knowing why you're not taking the call is valuable information to the caller.