The answer to the crossword for the term OFFICE PHONE LINE ABBR. is EXT, which stands for extension. An office phone line extension is a number that is added to an existing phone line in order to create a separate line for a specific purpose. This could be used to create a direct line for a specific department or employee, or to create a line for a specific type of call. Extensions are often used in businesses to help streamline communication and make it easier to reach the right person.