下面是一个示例函数，使用MySQL语法，它将接受未格式化的电话号码并返回格式化的电话号码列表：

CREATE FUNCTION format_phone_numbers(phone_list VARCHAR(255)) 
RETURNS VARCHAR(255)
BEGIN
  DECLARE delimiter_char VARCHAR(1) DEFAULT ",";
  DECLARE num_list_len INT DEFAULT 0;
  DECLARE output_list VARCHAR(255) DEFAULT '';

  SET num_list_len = LENGTH(phone_list) - LENGTH(REPLACE(phone_list, delimiter_char, '')) + 1;

  IF num_list_len = 0 THEN
    RETURN '';
  END IF;

  WHILE num_list_len > 0 DO
    DECLARE phone_number VARCHAR(20) DEFAULT '';
    DECLARE next_delimiter INT DEFAULT 0;

    SET next_delimiter = LOCATE(delimiter_char, phone_list);

    IF next_delimiter = 0 THEN
      SET phone_number = TRIM(phone_list);
    ELSE
      SET phone_number = TRIM(SUBSTRING(phone_list, 1, next_delimiter - 1));
      SET phone_list = SUBSTRING(phone_list, next_delimiter+1);
    END IF;

    IF LENGTH(phone_number) = 10 THEN
      SET output_list = CONCAT(output_list, FORMAT('%s-%s-%s', 
                      SUBSTRING(phone_number, 1, 3), 
                      SUBSTRING(phone_number, 4, 3), 
                      SUBSTRING(phone_number, 7)));
    ELSEIF LENGTH(phone_number) = 7 THEN
      SET output_list = CONCAT(output_list, FORMAT('%s-%s', 
                      SUBSTRING(phone_number, 1, 3), 
                      SUBSTRING(phone_number, 4)));
    END IF;

    IF num_list_len > 1 THEN
      SET output_list = CONCAT(output_list, ',');
    END IF;

    SET num_list_len = num_list_len - 1;
  END WHILE;

  RETURN output_list;
END;

要使用此函数，请执行以下语句：

SELECT format_phone_numbers('1234567890,1234567,5551234567') AS formatted_numbers;

结果应该为：`123-456-7890,123-4567,555-123-456