Leased Line
These are permanently connected communications links, dedicated to your company's exclusive use. If your Internet connection is very busy, or you would like permanent telephone connections between different offices (making the other offices' telephones act as if they were local telephone extensions), you might think about having a leased-line connection. This provides relatively secure, point-to-point connections between your offices and the Internet, dedicated solely for the use of your company. If the connections are business-critical, ISDN backup can be made available for use in the event of system failures. However, leased-line connections are considered to be the most reliable of all telecommunications links.
Virtually every major corporation in the world has leased-line connections or their local equivalent. The cost is approximately proportional to the speed you require (technically, the 'bandwidth') and, if you are connecting offices together, the distance between them. Leased lines usually work out to be cost-effective if you are connected to the same sites for long periods, say 5-6 hours a day or more, for data transfer or because your staff need to be on the phone to other sites for long periods of time, or just very frequently.