The plain old telephone system doesn't handle large quantities of data, and the phone companies realized this a long time ago. So the ISDN spec was hammered out in 1984 to allow for wide-bandwidth digital transmission using the public switched telephone network. Under ISDN, a phone call can transfer 64 kilobits of digital data per second.

Service	Dedicated	Price/month
ISDN 128k Personal Service	No	$39.95
ISDN Subnet 128k    "Business Hours"    12/hr. per day connection	Yes	$200.00
ISDN Subnet 128k    24/hr. per day connection	Yes	$300.00