add_filter( 'gform_field_validation', 'validate_phone', 10, 4 );
function validate_phone( $result, $value, $form, $field ) {
    $pattern = "/^(\+44\s?7\d{3}|\(?07\d{3}\)|\(?01\d{3}\)?)\s?\d{3}\s?\d{3}$/";
    if ( $field->type == 'phone' && $field->phoneFormat != 'standard' && ! preg_match( $pattern, $value ) ) {
        $result['is_valid'] = false;
        $result['message']  = 'Please enter a valid phone number';
    }
 
    return $result;
}