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IN CUSTOMIZER.PHP or FUNCTIONS.PHP
==================================

    //register footer settings
    $wp_customize->add_section('footer_setting', array(
        'title' => __('Footer settings', ''),
        'priority' => 25,
    ));

    //Add settings for the phone number
    $wp_customize->add_setting('phone_number', array(
        'default' => __('Phone number', ''),
        'transport' => 'refresh',
    ));
    $wp_customize->add_control('phone_number', array(
        'label' => __('Phone number settings', ''),
        'section' => 'footer_setting',
        'settings' => 'phone_number',
        'type' => 'text',
    ));