Step 1 - Sign into your AdWords Account
Step 2 - Select the campaign or ad group that you would like to add call extensions to.
Step 3 - Click on the Ad Extensions Tab
Step 4 - In the 'View' menu on the left hand side, select 'Call Extensions'
Step 5 - Click on '+Extension'
Step 6 - Click on +New phone number
Step 7 - Select the relevant country and enter in your call tracking number
Step 8 - Select 'Off' for Call Reporting
Step 9 - The below fields are optional to complete:
Step 10 - Click Save
Repeat the above process for every campaign/ad group you are adding a call extension number to.
Please Note: if you want to use call extensions, make sure you do not choose the location extensions feature. This will conflict with your call extensions number and they will not get approved by Google.