The function of the private line eliminates the possibility of someone other than you reviewing your call details, from phone sets, which have the shared extension, additionally no one can monitor your call or connect to you call.
Telephone service companies have created a special communication service without the need to use customer's equipment. It's called a "virtual number". It also has a text type of "virtual SMS center." Thanks to the virtual link you can make calls or send messages while connected to the equipment of the company.