I can show you two ways to add your customer's phone number and email address on invoices and estimates.
Firstly, you'll want to add these details to the Billing address section on your customer's information. This way, they'll be displayed when creating, printing, and sending the invoice or estimate. Here's how:
Click Sales from the left menu, then select Customers.
Choose each customer from the list.
Select Edit.
In the Customer information window, go to the Address tab.
Enter the email address and phone number in the Billing address field.
Click OK.
Secondly, you can also set up two custom fields for the email address and phone number. Then, manually enter the details when creating the invoice or estimate. Let me guide you how.
Click the Gear icon, then select Account and Settings.
Select Sales from the left menu.
Go to the Sales form content section.
Turn on the Custom fields option, then check two boxes and name them Email address and Phone number.