user centered design – Best way to ask for “This Or That Or Both” on a Form

I have a page where the User can search for it’s customers details based on multiple criteria.
Following screenshot is from one of the criteria:

Here First Name and Last Name are mandatory.
But the user is expected to enter either DOB OR Phone OR both.
What’s the best way to show that on the UI instead of adding a label.

  • for mandatory fields would mean user can try to search by entering only these 2 fields (first and last name).
    screenshot of search criteria