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//  $id = $this->GetVariable('LEAD_ELEMENT_ID_UPDATE');
$id = '2';

if ( !\Bitrix\Main\Loader::IncludeModule('crm') )
{
   return '';
}

try
{
   $id = intval($id);

   if ( empty($id) )
   {
      throw new \Exception("Lead id is empty");
   }

   $update = [
      'PHONE' => []
   ];

   $rs = \CCrmFieldMulti::GetList(
      [],
      [
         "ELEMENT_ID" => $id,
         'TYPE_ID'    =>'PHONE',
         'ENTITY_ID'  => \CCrmOwnerType::LeadName
      ]
   );

   while( $ar = $rs->fetch() )
   {
      // contvert value
      $value = '1'.$ar['VALUE'];
       
      $update['PHONE'][ $ar['ID'] ] = [
         'VALUE_TYPE' => $ar['VALUE_TYPE'],
         'VALUE'      => $value
      ];
   }

   $multi = new CCrmFieldMulti();
   $multi->SetFields( \CCrmOwnerType::LeadName, $id, $update);

}
catch( \Exception $e )
{
   \CCrmEvent::Add([
      'ENTITY_TYPE' => \CCrmOwnerType::LeadName,
      'ENTITY_ID' => $id,
      'EVENT_NAME' => 'Ошибка форматирования номера',
      'EVENT_TEXT_1' => '',
      'EVENT_TYPE' => '1'

   ], false);
}