$fields['billing']['billing_sim_phone_dialcode'] = array(
        'label'       => __('Select Dial Code', 'woocommerce'),
        'placeholder' => _x('', 'placeholder', 'woocommerce'),
        'class'    => array( 'form-row-first' ),
        'required'    => false,
        'clear'       => false,
        'type'        => 'select',
        'options'     => array(
            'option_a' => __('option a', 'woocommerce' ),
            'option_b' => __('option b', 'woocommerce' )
            )
        );