Hey there, @Great-Dane.
We can manually enter the customer's phone number in the Shipping address, Memo, or in Your message to the supplier fields. You can use them since the phone number saved in the customer's profile will not show up in the purchase order.
To add this to the purchase order, just go to the +New icon and select Purchase Order. Then, add the phone number in either of those fields.
In addition, you can also run purchase order reports in QuickBooks Online (QBO). This will allow you to keep tabs on outstanding orders and get an overview of your purchase history.
That should do it! Stay in touch with us here in the Community. Just click the reply button below so we can further assist you.