Calling someone without first getting their permission is pushy and annoying. Even though many of us secretly want to get calls from specific people, we wouldn't dare make one.
A single availability status for everyone you know is not how people deal with each other in real life.
Life is filled with stuff to do, even when we want to call someone, sometimes we forget.
Life has ups and downs, sometimes we can all use a little attention.
We used to get excited when the phone rang. Remember those days?
Phone calls create stronger bonds between people.