How can I alter the value of a field in Drupal 8?

I’m still trying to get used to Drupal 8 but i have a field that displays telephone numbers. When i display it it shows up at 1234567890 instead of (123)456-7890. 

Do I need to use a template hook to change the value before being displayed or do i alter the display in the field twig template?

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/22/feed