code: private function addRecipient($recipient)
      {
          if (is_null($recipient)) {
              throw new InvalidArgumentException('Recipient cannot be NULL');
          }
  
          if (!is_string($recipient)) {
              throw new InvalidArgumentException(
                  sprintf(
                      'Recipient must be presented as string, but "%s" given',
                      is_object($recipient) ? get_class($recipient) : gettype($recipient)
                  )
              );
          }
  
          if (\preg_match('/^\+\d+$/Ds', $recipient) != 1) {
              throw new InvalidArgumentException(
                  sprintf(
                      'Recipient must match pattern \+\d+, but "%s" does not match',
                      $recipient
                  )
              );
          }
  
          array_push($this->recipients, $recipient);
      }
docs: @param string $recipient A phone number in the one of formats: "+71234567890" or "71234567890".
  
  @throws \InvalidArgumentException If $recipient is not a string or format is invalid.