If you want to apply conditional validation on 'Phone Number' or 'Fax Number' then you need to add this input greater than or equal to '25'.
{{userForm.controls?.age?.errors?.digit.message}}
If you want to apply conditional validation on 'Phone Number' then you need to add 'Age' field greater than or equal to '25'.
If you want to apply conditional expression of type 'string'
{{userForm.controls?.faxNumber?.errors?.digit.message}}
Shows custom message
{{userForm.controls?.mobileNumber?.errors?.digit.message}}