Sure, but so then is email. I don't know if there is a perfect way to automate out of this problem, just providing the phone user a set of tools to judge whether to answer a call or not.

My own personal tool is: unless I am expecting a call from a strange number, callers not in my contact list are silenced automatically and go straight to voicemail. If it's important, like a relative calling from someone else's phone in an emergency situation or say, my bank calling about a fraudulent credit card charge, they will leave a message I can review. Especially with the voicemail transcription tools I get with my phone and provider, its pretty easy to delete the 'You won a free cruise' or 'IRS is suing you for fraud' messages.