I've been using Google voice since June 2007 when it was Grandcentral.com Google made the briliant move to purchase a company that makes a lot of things very simple.
You can screen callers, listen in before taking a call, also if needed record calls, even better block unwanted numbers. It also transcribes messages. Also the unique feature of ringing all of your phones at once.
you can find out more here
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go here to request an invitehttps://services.google.com/fb/forms/googlevoiceinvite/