Line is for the actual modem and is connected to the phone socket on your wall.
Telephone is for plugging in a telephone, answering machine, fax machine etc into an additional socket in an adaptor
Unfortunately using the Telephone socket can in fact cause more problems than they are worth
"Man's mind once stretched by a new idea, never regains its original dimension." - Oliver Wendell Holmes
Thank you for a very clear and prompt reply to my questions. However, I am still mystified because on one of the computers in the house we have attached the telephone line to the LINE socket, and on the other to the TELEPHONE socket, and both dial up and connect to the ISP