I'd like to make sure we're on the same page. Are you referring to the mobile phone number entered in the Customer Information? If so, adding the phone number field on sales transactions (quotes, invoices, sales receipts) is currently unavailable.
As a work around, add the phone number in the billing address section, or shipping address box.
Go to Sales > Customers.
Select the customer's name and click the Edit button.
In the Customer Information window, add the phone number above the street address information under the Billing address section. If you want the phone number listed at the bottom, you can use the Country field within the billing address section. This means that reports will show phone numbers in this field instead of their country.
If the Shipping address is different, you can use the same steps to add the phone number to the shipping address field.