The main method of joining WANs together is via the telephone system through Data Service Providers e.g BT, PIPEX. 

For example, a company might hire a 2MPs (mega bit per second) leased line from BT in order to allow the two networks to share information.  Obviously the higher the data rate that a business wishes to use, the higher the cost they must pay.  So it is a balance between their requirements (i.e. speed) and the price they wish to pay.

Typical leased line data rates:

There are alternatives to renting a digital data line and the next few pages describe the choices that may be made.