You Need to use ACF Options Page plugin as well.
This Plugin has been deleted from the repository. Now you can use the function without the plugin.
The options page feature provides a set of functions to add extra admin pages and all data saved on an options page is global.
Sea the plugin overview
After installing plugin you have to add the following to your functions.php
if( function_exists('acf_add_options_page') ) {
acf_add_options_page();
}
and the “Options” label will be visible in your admin area.
then you just need to create acf fields with the rule like 'If Options Page is equal to Options'
Finally to display the field:
echo get_field('phone_number', 'option');