올려주신 코드 중 아래의 코드를 찾아서

<div class="attr-value">
	<input type="text" id="phone-number1" name="kboard_option_<?php echo esc_attr($field['meta_key'])?>1" class="required" onKeyup="inputMoveNumber(this);" minlength="2" maxlength="3">&nbsp;-&nbsp;
	<input type="text" id="phone-number2" name="kboard_option_<?php echo esc_attr($field['meta_key'])?>2" onKeyup="inputMoveNumber(this);" minlength="2" maxlength="4" />&nbsp;-&nbsp;
	<input type="text" id="phone-number3" name="kboard_option_<?php echo esc_attr($field['meta_key'])?>3" onKeyup="inputMoveNumber(this);" minlength="2" maxlength="4" />
</div>

아래의 코드로 교체해보시겠어요?

<div class="attr-value">
	<input type="text" id="phone-number1" name="kboard_option_<?php echo esc_attr($field['meta_key'])?>1" class="required" onKeyup="inputMoveNumber(this);" value="<?php echo $content->option->{$field['meta_key'].'1'}?>" minlength="2" maxlength="3">&nbsp;-&nbsp;
	<input type="text" id="phone-number2" name="kboard_option_<?php echo esc_attr($field['meta_key'])?>2" onKeyup="inputMoveNumber(this);" value="<?php echo $content->option->{$field['meta_key'].'2'}?>" minlength="2" maxlength="4" />&nbsp;-&nbsp;
	<input type="text" id="phone-number3" name="kboard_option_<?php echo esc_attr($field['meta_key'])?>3" onKeyup="inputMoveNumber(this);" value="<?php echo $content->option->{$field['meta_key'].'3'}?>" minlength="2" maxlength="4" />
</div>

고맙습니다.

add_filter('kboard_skin_fields', 'my_kboard_skin_fields8', 10, 2);
function my_kboard_skin_fields8($fields, $board){
	
	if($board->id == '1'){ // 실제 적용될 게시판 ID 값으로 변경해주세요.
		
		if(!isset($fields['phone-number'])){
			$fields['phone-number'] = array(
				'field_type' => 'phone-number',
				'field_label' => '연락처',
				'class' => 'kboard-attr-text',
				'hidden' => '',
				'meta_key' => 'phone-number',
				'field_name' => '',
				'permission' => '',
				'roles' => '',
				'default_value' => '',
				'placeholder' => '',
				'required' => '',
				'show_document' => '',
				'description' => '',
				'close_button' => 'yes'
			);
		}
	}	
	return $fields;
}

add_filter('kboard_get_template_field_html', 'my_kboard_get_template_field_html8', 10, 4);
function my_kboard_get_template_field_html8($field_html, $field, $content, $board){
	
	if($field['field_type'] == 'phone-number'){				
		ob_start();
		?>			
		<div class="kboard-attr-row meta-key-<?php echo esc_attr($field['meta_key'])?> required">
			<label class="attr-name" for="phone-number">
            <span class="field-name">연락처</span>
            <span class="attr-required-text">*</span>
            </label>
			<div class="attr-value">
				<input type="text" id="phone-number1" name="kboard_option_<?php echo esc_attr($field['meta_key'])?>1" class="required" onKeyup="inputMoveNumber(this);" minlength="2" maxlength="3">&nbsp;-&nbsp;
				<input type="text" id="phone-number2" name="kboard_option_<?php echo esc_attr($field['meta_key'])?>2" onKeyup="inputMoveNumber(this);" minlength="2" maxlength="4" />&nbsp;-&nbsp;
				<input type="text" id="phone-number3" name="kboard_option_<?php echo esc_attr($field['meta_key'])?>3" onKeyup="inputMoveNumber(this);" minlength="2" maxlength="4" />
			</div>
		</div>
		<script type="text/javascript">
		function inputMoveNumber(num) {
			if(isFinite(num.value) == false) {
				alert("연락처는 숫자만 입력할 수 있습니다.");
				num.value = "";
				return false;
			}
			max = num.getAttribute("maxlength"); // 3글자 쓰면 자동으로 다음 칸 넘어가기 
			if(num.value.length >= max) {
				num.nextElementSibling.focus();
			}      
                    
		}
            
                  
		</script>
		<?php
		$field_html = ob_get_clean();
	}
	
	return $field_html;
}

admin-document.php 에는 <?php echo $content->content?> 아래 넣었습니다.

<div class="kboard-content" itemprop="description">
				<div class="content-view">                 
					<?php echo $content->getDocumentOptionsHTML()?>
					<?php echo $content->content?>
                    <p>연락처 : <?php echo $content->option->{'phone-number1'}?>-<?php echo $content->option->{'phone-number2'}?>-<?php echo $content->option->{'phone-number3'}?>
                    </p>
				</div>
			</div>

추가하신 코드만 에디터의 코드 스니펫 삽입 기능으로 올려주시겠어요?

고맙습니다.