Thanks for joining this conversation, ThePlantGuys.

While the option to automatically include a customer's phone number on an invoice is currently unavailable in QuickBooks Online (QBO), I know a workaround that will lessen your work to achieve this goal.

Instead of adding a custom field and entering their phone numbers manually, you can include this information on the customer's billing/shipping address. 

Here's how:

1. Go the Sales menu at the left pane to get to the Customers page.
2. Click the name of the customer to open their information.
3. Select the Customer Details tab, then tick Edit. 
4. Enter the phone number in the Billing/Shipping address section.
5. Hit Save. 

Once done, it will automatically show in the invoice after selecting their name. 

Just in case, I'll be adding this article for future reference: Edit customer information.

I'll pass along your suggestion to our Product Developers. They're always looking for ideas from users on how to improve QBO.  In the meantime, I recommend visiting our Blog site. This is where we share recent happenings and future developments, such as updates to newly added features. Here's the link: https://quickbooks.intuit.com/blog/.

I want to make sure that everything is set, so please let me know if you have any other issues or concerns. I'll get back to you as soon as possible. Have a great day!