Where Can I Find the Syntax For Adding A Phone Number Field to The Checkout Page?
If you want to collect the customer's phone number but make it optional you can use the isRequired attribute as follows:
[MM_Form_Field type='input' name='phone' isRequired='false']
When you include it like this, the customer can optionally enter in their phone number. You can always insert the template for referencing as well. You can read more about using the templates here: http://support.membermouse.com/support/solutions/articles/9000074880-use-the-core-page-templates