I have a phone list in a works spreadsheet, I have ...

You'll need a sheet with customer details in with the phone number as the first column.  On the tracking sheet you can make the name field into a vlookup so that it returns the name once the phone number has been entered.  Rather than typing in the numbers on the tracking sheet all the time, you would probably be better off using Data validation to give you a drop down of all the numbers you have to choose from.  Then it would just be a case of scrolling down the list to find the number and once selected the name would appear as well.