To add a phone number to your header, go into your header template. Click into the row, then click the hamburger icon in the yellow tooltip to add a new column (as shown in the image below).
From there, you can type the phone number into the new column and move it to the desired section. At the top, you can also change the text from a paragraph to the appropriate heading tag and style it as needed.
Do you have the Crio Pro plugin installed and active on your website? This is how you can create custom page headers and add the phone number where you want. Please let me know.