All those long data calls place a burden on the central switching equipment. A modem may take a minute to dial the line and establish a connection. An ISDN connection can be established in seconds. Many ISDN devices will hang up the line when there is no traffic, then dial back automatically when there is something to send. And with a digital connection, there is no meaningless noise continuously flowing between the two ends.
Bytes flow only when there is meaningful data.
ISDN could be a win-win arrangement for the phone companies. They can install two phone lines in the same wire now used for a single line. ISDN uses the central office equipment more efficiently, and may limit the need for expensive upgrades of switching equipment to handle the increasingly popular internet services. Unfortunately, local phone companies have viewed ISDN as a premium service for which they could charge an unreasonable price.