I would actually suggest to think in reverse. Start by asking the question why it is important to tell or show the user that they are only required to fill in one of the two options. Especially when filling in both options is completely fine, too.
The average user has already filled in hundreds of forms just like this one during their life time, so most of the process is somewhat automatic. By adding the extra condition that the user has to fill in at least one of the options, are we not making the form unnecessarily more complex? Could it be that the form would be easier and faster to fill in without this extra condition?
The scenario as I see it:
A) Please fill in these 4 fields.
B) Please fill in these 2 fields, and at least one of the other 2, or both.
By trying to give the user the choice of leaving one field empty, maybe we are adding to their cognitive load of having to try and understand what is expected from them?
One suggestion I would propose is to keep both fields (DOB, Phone) as optional fields, and only inform the user to fill in at least one of the fields when they try to submit the form with both of these fields empty. This way the condition is only shown when relevant, and at a time when it benefits the user more: "Ah, I only have to fill in one of these, then I'm done".
Another suggestion would be to place the condition at the top of the form. If you move the DOB and Phone field to the top and provide visual feedback, for example by adding "OR" in between input fields, it is instantly clear to the user that they have to fill in (at least) one of these fields, and everything else follows from that.
This suggestion is inspired by many online platforms that require at least your email or phone number or (less common nowadays) a username. Your account ID and information in the database is based on one of those pieces of information and everything else (your account info, data, etc.) will be based on your email, phone or username. As a result, you could then reset your password by giving the system your email, phone, username or a combination of the three, depending on their requirements.
The point of this suggestion is to use as much familiarity with other (digital) systems to make the process as intuitive as possible.