Error validating the formula
To add the testing for the proper characters, we’ll need to wrap the phone number validation formula in an IF() statement and replace the second TRUE result with it. With that formula, our list doesn’t require a phone number to be entered, but if one is entered, it must be in the proper format.
To allow entries that contain specific text anywhere in a cell (in the beginning, middle, or end), use the ISNUMBER function in combination with either FIND or SEARCH depending on whether you want case-sensitive or case-insensitive match: The formulas work with the following logic: You search the substring "AA" in cell A2 using FIND or SEARCH, and both return a position of the first character in the substring. For any numeric value returned as the result of search, the ISNUMBER function yields TRUE, and data validation is successful.However I want to validate the entry if/when it is filled in.Is there a way to add string at end of the phone validation formula to accept a ‘null’ value to avoid validation error?Below you will find a few examples of custom validation rules for different data types. All Excel data validation rules, built-in and custom, verify only new data that is typed in a cell after creating the rule.Copied data is not validated, nor is the data input in the cell before making the rule.