You can create a custom field for your customer's contact number, @Enzo. I know how to do this. Let me share it with you.

To display your customer's contact number when printing an invoice, I suggest adding a custom field to your sales forms. To proceed, here's how:
 

1. In the Gear icon, select Custom fields under the LISTS column.
2. Click Add Custom filed. If you already created a field, it will show the Add field.
3. Put a name for that field in the Name. (Example: Customer's contact number)
4. To show the added custom field when printing, click the Print on form.
5. Once done, hit Save.

In addition, I suggest providing feedback to our development team requesting the inclusion of a customer's contact number on invoices without a custom field. Know that our developers are working hard to improve our product to meet our customers' needs. To send a message, use the following steps:
 

1. Select the Gear icon, then choose Feedback.
2. Type in your comments or product suggestions.
3. Select Next to submit.

On the other hand, you can also personalize your sales forms to fit your business-looking standards. To know more, check this article: Customize invoices, estimates, and sales receipts in QuickBooks Online.
After managing invoices, let us know if you have additional queries about any QuickBooks-related. We will be happy to assist. Take care!