Thanks for choosing QuickBooks Online to back your business. It's an awesome option for many business owners that keeps bookkeeping easy so you can do more of what you love with your job. Keeping track of customers can be tricky when you have a large list of them, and I'm happy to share there are a few simple ways to search for your customers by phone number.
The first and perhaps easiest way to accomplish this is by clicking the magnifying glass icon at the top right of your account and then typing the phone number into the search bar. When you do this, the program will populated with information that uses that number, for instance, the customer, and give you a clickable option to navigate to the customer profile. Check out this example from a test account.
You can also search the customer by phone number on the Customers page accessed through the Sales menu on the left. Here's another image for you to see it in action.
Another useful tool for seeing your customer information is a report called Customer Contact List. Simply click the Reports tab on the left, and then either scroll through the options or use the search bar Find a report by name to find the one I just mentioned. While it doesn't quite give you the option to search like a search bar would, it provides a comprehensive list of all the customers in your account.
I hope that helps! Give me a shout if there's anything else I can do for you.