Thank you for adding a post here on the Community page, @hdsds19.

I'm here to share some insights about the customer information in QuickBooks Online (QBO).

If you're trying to add the customer's mobile or phone number, you can enter the details under the Phone or Mobile section from the Customer information page. 

Here's how:

1. Open your QBO account, then select Customer from Sales at the left pane.
2. Click on the customer's name to open it.
3. Select Edit beside New transaction.
4. Type in the info under Phone or Mobile, then save.

However, if you're referring to the number of the customer from your record (order from the list), you can add it beside the display name. If you want to use the display name when printing the customer's checks, mark the box beside Print on check as display name.

In case you need an additional reference when managing your customers in QBO, feel free to open this link about how to add customers, sub-customers, delete or merge customers.

Also, open read our help articles for more tips while working with QuickBooks in the future.

I'm always here to help if you have any other QuickBooks questions. Mention my name and leave a comment below. Keep safe, @hdsds19!