Hi there, @morethanmulchsup. 

At the moment, the option to add a phone number on the invoice in QuickBooks Online is unavailable. You'll want to add the phone number in the billing address section or add it directly to the other sales information. 

Here's how:

1. Go to Sales, then select Customers.
2. Choose a customer's name, then select Edit.
3. In the Billing address section, add the phone number above the street address information. 
4. If the Shipping address is different, use the same steps to add the phone number to the shipping address field.
5. Select Save.

You can check out this article to learn more about adding a customer's phone number to the sales transaction.

The Community is always here to help if you have any other questions.