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.
Concerning practical implementation, the VoIP gateways have proven to be the best. Many companies are successfully using the IP telephony thanks to its two main characteristics
Decrease in international and long distance call cost expenditures
Stable communications between company’s offices throughout different cities and counties