Several different vendors have incompatible versions. ISDN Digital phone equipment is used everywhere except at your home or office. Since the limit on data transmission is caused by the conversion of sound to a digital signal, the obvious direct solution to the problem is to extend digital signaling all the way to the home. A T1 line would provide this type of connection. It carries 24 phone circuits each with 8000 bytes per second, or a single circuit of around 1.5 Million bits per second.
Prices vary depending on location.
An international standard called ISDN has been established for an less expensive personal digital service. This is called a Basic Rate Interface or BRI circuit. Because it is designed for use in a home or small office, ISDN allows the lines to be shared by voice telephones, fax machines, and computers. Only the fax machine would answer an incoming fax. An incoming voice call would ring the phone.