Creating custom shortcode

You are passing the parameters wrong, try with something like this:

function get_telephone($attrs) {
$attr = shortcode_atts( array(
      'name' => 'John Doe'
  ), $attrs );
return attr['name'];
}
add_shortcode( 'telephone', 'get_telephone' );

In the post you call the shortcode like this: [telephone name="Jose"]

Anyway, inside the get_telephone function you should add some logic to get the phone numbers from somewhere, it could be an array, or you could create a custom post type to store the numbers.