Example of how to override the validation stratey used on a EE_Phone_Input field.

ee_phone_input_validation.php

<?php //Please do not include the opening PHP tag

add_filter('FHEE__EE_Form_Input_Base___construct__input_args', 'tw_ee_EE_Form_Input_Base___construct__input_args', 10, 2);
function tw_ee_EE_Form_Input_Base___construct__input_args( $input_args, $input ) {
    if( $input instanceof EE_Phone_Input ) {
        if( $input->has_validation_strategy('EE_Text_Validation_Strategy') ) {
            $input->remove_validation_strategy('EE_Text_Validation_Strategy');
            $input->add_validation_strategy(
                new EE_Text_Validation_Strategy(
                    __('Please enter a valid phone number. Eg 123-456-7890 or 1234567890', 'event_espresso'),
                    '~^(([\d]{15})|(^[\d]{3}-[\d]{3}-[\d]{4}))$~'
                )
            );
        }
    }
    return $input_args;
}