1. In your WordPress dashboard, go to Appearance → Menus page.
2. Make sure you’ve created a custom menu for your site. Otherwise, you will not be able to add a Link to the menu. To create a custom menu, click on create a new menu link. Then add a Menu Name and click on Create Menu button.
3. After that, click Custom Links in the left section to add a phone number link to WordPress navigation menu.
4. Remove the http:// in the URL field, and add your phone number using the following format: tel:+number. Then enter your text to the Link Text field.
5. Click on Add to Menu button to place your link to the menu.
6. Finally, click on Save Menu button to save your changes.
I hope that helps, let us know if you need further assistance.