You can customize the template to add the customer number manually, SusannaR.

Please refer to KhimG response for the steps. Once added, you'll now have the option to add the number for all future invoices. Just make sure to use the correct invoice template.

Editing each customer's billing address to add the phone number is optional. Doing this will let you see the customer's number in the invoice automatically.

You might want to check out this article for your reference: How to customize invoices, estimates, and sales receipts.

Get back to me if you need help with anything else. I'll be here to help you some more. Thanks.