Many different types of telephone lines are available, but all the choices break down into two groups: dedicated and dial-up. Dedicated lines are always off the hook (that is, they never hang up on each other). They are always on.
A dedicated line (like a T1) does not have a phone number. In essence, the telephone company creates a permanent, hard-wired connection between the two locations, rendering a phone number superfluous. Dial-up lines, by contrast, have phone numbers; they must dial each other up to make a connection. When they’re finished communicating, they hang up. Two technologies make up the overwhelming majority of dial-up connections: PSTN and ISDN.
Public Switched Telephone Network
The oldest, slowest, and most common ...