Remove Dokan seller registration form default validation and add new validation.

functions.php

// remove the filter 
remove_filter( 'woocommerce_process_registration_errors', 'dokan_seller_registration_errors' ); 
remove_filter( 'registration_errors', 'dokan_seller_registration_errors' ); 

// New registration form erros handling
function dokan_update_seller_registration_errors( $error ) {
    $allowed_roles = apply_filters( 'dokan_register_user_role', array( 'customer', 'seller' ) );

    // is the role name allowed or user is trying to manipulate?
    if ( isset( $_POST['role'] ) && !in_array( $_POST['role'], $allowed_roles ) ) {
        return new WP_Error( 'role-error', __( 'Cheating, eh?', 'dokan' ) );
    }

    $role = $_POST['role'];

    if ( $role == 'seller' ) {

        $phone = trim( $_POST['phone'] );
        if ( empty( $phone ) ) {
            return new WP_Error( 'phone-error', __( 'Please enter your phone number.', 'dokan' ) );
        }
    }

    return $error;
}

add_filter( 'woocommerce_process_registration_errors', 'dokan_update_seller_registration_errors' );
add_filter( 'registration_errors', 'dokan_update_seller_registration_errors' ); 

'remove_filter' not working