Let me start by saying that you have ressurected a 3 year old topic. You should have read the warning and let it be. Especially when you don't really know what you are talking about.
The plus symbol (+) has 2 functions: a) to add when used with numbers b) to concatenate when used with strings.
The post that you've commented includes a space within quotes, so at best it will produce an error if the Tel_H_Code or Tel_H is a number. If they have been declared as chars then it will just concatenate them.
This worked to combine 2 string values (first name & last name) from 2 separate columns (first_name, last_name) into 1 new column (full_name). Straight out of the textbook: SQL Fundamentals 3rd edition pg. 344:
select employee_id, first_name || ' ' || last_name AS full_name, hire_date from l_employees;