I want to select an account with specific phone number or its primary contacts phone number is equal to the given number
In this scenario we may not able to form an or condition for the filters as the filters are split between the parent(account) and the linked entity(contact) in advanced find editor. But the same can be achieved in fetchxml.
Fetchxml supports conditions from other linked entities, the trick is to use an attribute called entityname in condition node to specify which entity it links to. The same is highlighted below.
Hope this helps!