Why doesn't the customer phone number print on estimates, invoices, and sales receipts?

Hello there, @n-tejeda-newdime.

In QuickBooks Online (QBO), the customer's phone number that we set up in the Customer Center does not transfer over to the sales transactions (estimates, invoices, or sales receipts). 

As a workaround, you may either include the phone number in the billing address area or, if you only want it to appear on a particular transaction, add it directly to the the sales form when creating.

To add a phone number permanently to sales transactions:

1. Go to the Sales menu then select Customers.
2. Locate the customer the hit on Edit.
3. Scroll down to the Billing Address section and add  the phone number in either which of the field. 
4. When done, click Save.

Now when I create a sample invoice, the phone number already shows.

To add a phone number at a single transaction:

1. Open the sales transaction you want to edit.
2. Select Billing address then type in the customer's phone number. 
3. Select Save.

Additionally, I've included an article that'll help you utilize the Custom transaction numbers option to set your own numbering preference on your sales forms: Add custom transaction numbers to sales forms in QuickBooks Online.

Fill me in if you have more queries with regard to managing your sales forms. I'd be glad to assist you further. Keep safe.