Find the code for your dynamic number in the "My Numbers" section of your company, and click over on the "Code Insertion" tab. If you have questions on where to find this - please follow this guide. Also, be sure the Google Tag Manager is installed on the site you are implementing the dynamic code on before beginning.
Log into Google Tag Manager and navigate to the domain of the site you are inserting the number.
Click New Tag.
Name the tag at the top of the page, then select the box labeled Tag Configuration.
Select Custom HTML under the section Custom.
Copy your dynamic number code from to bottom of the Configure tab in My Numbers and paste it into the HTML box in Google Tag Manager.
Click the Triggering box below, and choose your trigger. We recommend setting your script to fire during loading of all pages of your website by selecting the default All Pages trigger.
Click Save to add your tag to your workspace.
You can test your tag by hitting Preview and then navigating to the url of your site. It is best to do this in an incognito browser to avoid being cookied. Make sure you see the dynamic number swapping in the scenarios that it should (i.e ‘All Traffic, only AdWords, from Facebook’ etc.).
Once your tag has been added to your workspace and you have tested, click Submit at the top of the page to activate the script on your website.