Thanks for getting back here, @FournierFinishing. Let me walk you through how you can add your customer's phone number and email addresses to QuickBooks Online (QBO).

We can use the custom fields feature in QBO to add a field in your sales forms and purchase orders. Here's how to create a new field:

1. Go to Settings ⚙, then select Custom fields.
2. Choose the Add field.
3. Enter a name in the Name field.
4. Select the All Sales forms checkbox.
5. If you want the custom field to appear on printed and delivered forms, turn on the Print on form switch. Otherwise, it will only be visible in QuickBooks.
6. Once done, select Save.

See the screenshot below for visual reference:

Importantly, if you still don't have this feature, please know that this feature is exclusive to the QBO Plus and Advanced versions.

Furthermore, you can use the Message on invoice or Message on statement field to add your customers' information.

Moreover, you can check out this article for another workaround to show your customers' phone numbers in your sales transaction.

Additionally, once everything is settled with your invoices, you can use this if your customers' have paid for your product or services: Record invoice payments in QuickBooks Online.

Please let me know how it goes or if you have any follow-up questions about this feature in QBO. I'll be here if you need further assistance. Have a wonderful day!